The next question on my list:
2. How does the religion relate to previous and subsequent religions?
If a religious prophet claims to reveal the God who created the universe... well, since God must have existed before that guy came along, that prophet should have some story to tell about what God was doing beforehand, prior to the time that his own religion came into existence! This suggests we should look for a religion that either (a) has roots going back to ancient history, or (b) is a plausible continuation of such religions.
Of course, just because a religion claims to originate from (or restore) some ancient religious tradition, doesn't necessarily mean that those claims are historically accurate! There are plenty of examples of modern religious movements that have fake origin stories. For example, certain Masonic texts describe an elaborate mythological origin for Freemasonry, involving Euclid, ancient Egypt and King Solomon's Temple, but there is no good evidence that the organization even existed before medieval times.
Similarly, Wicca claims to be based on an ancient pagan traditions, but actually has very little in common with any historical form of paganism. (Their myth that millions of Wiccans were killed during the "Burning Times" is therefore essentially fictitious, since Wicca did not exist then. The witch hunts of the 15th-17th centuries were on a much smaller scale, and most of those killed were falsely accused Christians.) Wicca actually originates with the 20th century occult figures Gerald Gardner and Doreen Valiente, who constructed a more friendly version of the magical practices of the Satanist Aleister Crowley, by replacing the explictly selfish aspects with more humane principles.
What real Paganism was like
However, in the ancient world there was such a thing as real paganism. Typically it involved the worship of a pantheon of mulitple gods and goddesses. Usually, the pagan myths described their gods as beings of finite power, wisdom, and goodness, who in turn produced new deities by various scandalous methods of sexual and asexual procreation. These gods did not create the entire universe, they just built certain aspects of it out of pre-existing materials.
These pagan religions did not usually regard themselves as making exclusive truth claims; there was plenty of warfare, but almost never over the question of whose pantheon was "real". When polytheists encountered a new form of worship, they seldom denied the existence of the new deity (although they sometimes concluded that the victors' gods must be more powerful). Often, they would identify the other culture's god with a corresponding deity in their own pantheon. Alternatively, they would simply add the new god to their own pantheon, and start worshipping it alongside the others. So the different polythistic religions should not necessarily be regarded as having rival worldviews. They are much more like variations of the same basic thing, like different local varieties of wine or cheese. In cosmopolitan cultures like the Roman Empire, they were freely exchanged to suit individual taste.
The Weirdness of Israel
In the middle of all this, there was something else, which by any objective standard was quite different from the rest. Israel was remarkable for being the only ancient nation whose national religion was strict Ethical Monotheism, eschewing all idolatrous representations of God in the visible form of any created being. Although many great philosophers from all nations have arrived at the understanding that there is one God who created everything, Israel claimed to have this knowledge by revelation, because God had rescued Israel from slavery and made a covenant with them to be his own special people, and so gave them a special responsibility to obey his Law that no other nation has:
You saw no form of any kind the day the Lord spoke to you at Horeb out of the fire. Therefore watch yourselves very carefully, so that you do not become corrupt and make for yourselves an idol, an image of any shape, whether formed like a man or a woman, or like any animal on earth or any bird that flies in the air, or like any creature that moves along the ground or any fish in the waters below. And when you look up to the sky and see the sun, the moon and the stars—all the heavenly array—do not be enticed into bowing down to them and worshiping things the Lord your God has apportioned to all the nations under heaven. But as for you, the Lord took you and brought you out of the iron-smelting furnace, out of Egypt, to be the people of his inheritance, as you now are. (Deuteronomy 4:15-20)
It is difficult to express just how unusual this prohibition of images was in its cultural context, but it allowed Israel to have a more sophisticated and less anthropomorphic notion of deity than the surrounding pagan cultures.
Another rather remarkable aspect of the Jewish religion is the large number of different prophets whose teachings were recorded in great detail. The different books of the Hebrew Scriptures were written over many centuries—about a thousand years from Moses to Malachi, if you take the biblical chronology at face value—and contain the records of dozens of different prophets, each proclaiming the words of the same Deity, with a broadly consistent message. This is quite remarkable, and I do not know of any parallel among polytheistic religions.
While there were indeed pagan prophets and oracles, you cannot buy the "collected writings of the prophets of Aphrodite" in a bookstore, or if you could it would be a very short book! Nor can you find an equivalent to the prophet Isaiah, in which Apollo speaks at length about his nature and his plans for human history. (The closest one gets is a few ambiguous scraps of advice attributed to the Oracle of Delphi.) These texts never existed, because these pagan gods simply did not reveal themselves in a definitive literary form, the way that the God of Israel does in the Bible.
Monotheism vs. Polytheism
So who is more likely to be right? The diverse majority, or the peculiar minority? Well, for one thing Polytheism does not seem to be very compatible with a scientific understanding of the world. Nature is impersonal; it can be studied and normally it seems to operate according to fixed and objective principles. Although intelligent beings more powerful than human beings might well exist, is hard for me as a scientist to take seriously the idea that different aspects of the Universe originate from a pantheon of squabbling deities. It makes more sense to say that the real source of all things is something more basic and unified: either a single Almighty Deity, or a single Law of Nature. So the a priori philosophical plausibility of Monotheism provides a solid reason to think that Israel is special. All by itself, this tells us that Judaism is more likely to be true than any other randomly selected ancient religion.
(Note that, even if one were to assign e.g. an 80% prior probability to the majority opinion of Polytheism, and a 20% prior probability to the minority opinion of Monotheism, that would still make Judaism WAY more probable than 1 / total # ancient religious cults. And that is before considering any specific evidence for supernatural or miraculous events, which we will get to in future questions.)
We Moderns would probably have preferred a more "democratic" setup, where all people groups are given equal access to divine revelation, and nobody is specially privileged. Well, that's just too bad, because there is no religion like that. Not unless we want to water down what we mean by religion to a lowest common denominator doctrine shared by multiple traditions (e.g. Ethical Monotheism or Pantheism) that removes all of the distinctive teachings. Some aspects of these lowest-common-denominators might be valuable and true, as far as they go. But if you want a religion that says anything nontrivial about God or humanity, then it has to be a specific religion, capable of making nontrivial claims that may contradict other religions. (Although they can agree with other religions about important aspects of the truth.)
Anyone who accepts Jewish-style Monotheism must accept that Paganism was, at the very least, deeply confused. However, that does not require us to say that it was entirely isolated from real spiritual experiences. If human beings have a natural instinct or intuition that God exists, it is quite possible that many pagans indirectly sensed God's presence in Nature, and wrongly associated this numinous experience with a more limited, localized concept of deity.
Mythology and Christ
Some Christians have noticed that, even though Paganism is incompatible with Jewish-style Monotheism, certain pagan myths can still be thought of as foreshadowing the Gospel in striking ways. Despite what some crackpots say, the stories about Jesus were not directly copied from pagan myths. However, elements such as e.g. gods being sacrificed to produce a harvest, or Odin hanging himself on a tree for nine days in order to gain wisdom, do hint at the method of salvation later provided through Christ. Christians can attribute this to the influence of the Holy Spirit preparing the nations to receive and understand the Gospel of Christ. Since human beings are created in the image of God, it is not surprising that human myths should in some way reflect our divine origins, as St. Tolkien argued in the conversation that resulted in St. Lewis' conversion:
“Dear Sir,” I said—Although now long estranged,
Man is not wholly lost nor wholly changed.
Dis-graced he may be, yet is not de-throned,
and keeps the rags of lordship once he owned:
Man, Sub-creator, the refracted Light
through whom is splintered from a single White
to many hues, and endlessly combined
in living shapes that move from mind to mind.
Though all the crannies of the world we filled
with Elves and Goblins, though we dared to build
Gods and their houses out of dark and light,
and sowed the seed of dragons—'twas our right
(used or misused). That right has not decayed:
we make still by the law in which we're made.
("Mythopoeia", as quoted in "On Fairy Stories")
Yet these pagan myths (which include all kinds of obscene, superstitious, and absurd elements) can in no way be regarded as parallel to the explicit revelation of God to the prophets of Israel. The teaching of Jesus presupposes that God really did reveal himself to Israel in a special way.
Christianity as the Fulfilment of Judaism
So if Christianity is true, then the essential claims of Judaism are also true. They are not necessarily rivals. Instead, Christianity claims to be the fulfilment of Judaism.
I find this claim of continuity plausible because Judaism is clearly marked as provisional: there are many prophecies of a Messiah, descended from King David, who will be an exalted figure that leads to a new era in which God relates to people in a more intimate way; they describe both his suffering and his later glory. The New Testament states that Jesus fulfilled many of these prophecies in specific and surprising ways, even though some of them are still waiting for his Second Coming.
There are several possible objections a Jew might make to these claims, but probably the two most important to address are these:
First, the Torah (the Law given through Moses, contained in the first 5 books of the Bible) states that it is eternal, whereas the New Testament explicitly abrogates many of the specific ceremonial decrees of the Torah, such as circumcision and kosher rules.
Of course, the New Testament agrees that the Torah sets up an eternally valid relationship between Israel and her Lord; and also that the Torah, being divinely inspired, is a perpetual source of wisdom and guidance in the Church. In Christianity the term "fulfilment" comes from Matthew 5:17-19, where Jesus agrees that the Torah has perpetual validity:
Do not think that I have come to abolish the Law or the Prophets [i.e. the Hebrew Scriptures]; I have not come to abolish them but to fulfill them. For truly I tell you, until heaven and earth disappear, not the smallest letter, not the least stroke of a pen, will by any means disappear from the Law until everything is accomplished. Therefore anyone who sets aside one of the least of these commands and teaches others accordingly will be called least in the kingdom of heaven, but whoever practices and teaches these commands will be called great in the kingdom of heaven.
Of course, the reason Jesus had to say that explicitly, is that in the rest of the chapter he asserted his authority to radically reinterpret the meaning of obedience to the Law, and he didn't want people getting the impression that he was rejecting the validity of the Torah as the words of God.
The question is whether you still have to obey all the ceremonial laws in a literal sense. And here the answer can be conclusively proven to be No just from the Hebrew Scriptures alone. For the prophets state that the Messianic age will result in a New Covenant being made, to replace the old one. If the sacrifices and rituals of the Old Covenant were still adequate, there would have been no talk about replacing them with a new regime in which things work differently. (To be truly consistent with a no-updating principle, one would also have to reject most of the prophets that followed Moses, like the Samaritans do.)
Also, the Torah itself indicates that the reason for many of its ceremonial rules is to keep the Jews separated from the Gentiles [non-Jews] and their immoral practices. For example:
“You must not live according to the customs of the nations I am going to drive out before you. Because they did all these things [sexual immorality and child sacrifice], I abhorred them... I am the Lord your God, who has set you apart from the nations. You must therefore make a distinction between clean and unclean animals and between unclean and clean birds... You are to be holy to me because I, the Lord, am holy, and I have set you apart from the nations to be my own.” (Lev. 20:23-36)
But one of the major predictions about the Messianic era is that the Gentiles will be joined with the Jews in the worship of the true God. Many examples could be given; here is just one:
Many peoples will come and say:
“Come, let us go up to the mountain of the Lord,
to the temple of the God of Jacob.
He will teach us his ways,
so that we may walk in his paths.”
The law will go out from Zion,
the word of the Lord from Jerusalem.
He will judge between the nations
and will settle disputes for many peoples.
They will beat their swords into plowshares
and their spears into pruning hooks.
Nation will not take up sword against nation,
nor will they train for war anymore. (Isaiah 2:3-4)
But if the Gentiles give up their idolatry and worship God together with the Jews, this implies that the special ritual observances which were designed to keep Jews and Gentiles separate, having fulfilled their stated purpose, ought to come to an end!
The point of what the prophets (and Jesus) are saying is that nothing in God's commandments can be rejected as spurious, but if any part of God's plan "has been accomplished" (in the sense of its purpose being fulfilled) then of course the proper way of proceeding will be different from that point on. Suppose you are driving your children to a party. You might tell them "Don't open the car doors while we are driving!", because they need to obey that commandment for their own safety, as long as its reasons are still applicable. But you can still tell them to "Get out of the car" once they've reached the destination, because arriving at the destination (with you) is why they were in the car to begin with! That is the difference between abolishing and fulfilling a command.
No other gods
A second and more critical objection, is that the whole point of the Torah was to teach Monotheism, but Christianity assails this doctrine by asserting the divinity of Jesus; that he is the Son of God and eternally one of the persons of the Trinity (along with the Father and the Holy Spirit). And the Jews were supposed to reject any prophet who told them to worship other gods:
If a prophet, or one who foretells by dreams, appears among you and announces to you a sign or wonder, and if the sign or wonder spoken of takes place, and the prophet says, “Let us follow other gods” (gods you have not known) “and let us worship them,” you must not listen to the words of that prophet or dreamer. The Lord your God is testing you to find out whether you love him with all your heart and with all your soul. It is the Lord your God you must follow, and him you must revere. Keep his commands and obey him; serve him and hold fast to him. That prophet or dreamer must be put to death for inciting rebellion against the Lord your God, who brought you out of Egypt and redeemed you from the land of slavery. (Deut. 13:1-5)
One possible response to this is that the Hebrew Scriptures already contain several hints that the Messiah would have some superhuman or quasi-divine status. He acts as a sort of eternal mediator between God and Israel, ruling over all the peoples:
Every warrior's boot used in battle
and every garment rolled in blood
will be destined for burning,
will be fuel for the fire.
For to us a child is born,
to us a son is given,
and the government will be on his shoulders.
And he will be called
Wonderful Counselor, Mighty God,
Everlasting Father, Prince of Peace.
Of the greatness of his government and peace
there will be no end.
He will reign on David’s throne
and over his kingdom,
establishing and upholding it
with justice and righteousness
from that time on and forever. (Isaiah 9:5-7)
The most important thing here is not so much the title "God" considered by itself, since the Hebrew word אֵ֣ל (el) is ambiguous and could also be translated as "judge" or "hero". Looking at the entire context is important. And in that context, the more essential point is that God is willing to trust a single human being with ruling all nations forever, saving us all from war and death, and acting in that capacity as a perpetual parent ("Eternal Father") and guide for the human race. But Monotheism has no room for a semi-divine savior figure like this, since there is only one God—and as he says "apart from me there is no Savior" (Isaiah 43:11). So it was actually to preserve the meaning of Monotheism (trusting in one God as the only source of salvation) that the early Christians decided that Jesus must be fully divine, of one being with his Father; and not a separate god from the God of Israel.
It is (once again) the fulfilment of what God was aiming at all along. Throughout the Hebrew Scriptures, God states that he will come and live among his people: through their observance of his Law, their reverence for his Name, in their Temple, etc. But none of these proved sufficient by themselves; people kept turning away. It is a dramatic and shocking claim (blasphemous unless it is true) that God came and dwelt among his people in a temple of human flesh. But it gives new meaning to all of these promises, which could never really be fulfilled by a mere building in Jerusalem.
Why was God not more explicit about this from the beginning? Because like all good educators, God was revealing the truth progressively, rather than trying to present all the material at once. (For example, it was not until the later stages of the Jewish revelation that prophets began to explicitly teach about an afterlife.) It took over a thousand years to drill Monotheism into Israel and prevent them from continually falling back into idol-worship, polytheism and other pagan practices. It was hard enough keeping the Israelites from having shrine prostitutes in the Temple, and sacrificing their own children to dark gods. During this time the doctrine of the Trinity could only have been a distraction, misunderstood in the grossest possible ways (e.g. as a pantheon generated by sexual procreation). First the Lord had to make it quite clear that he alone is God, that he is holy and righteous, and not to be depicted by an image or understood in a crudely anthropomorphic way. Only after these lessons sunk in, could the mystery of the Trinity be fully revealed (as opposed to merely hinted at).
Islam as Further Fulfilment?
Now Islam claims, in turn, to be a fulfilment of both Judaism and Christianity. Muslims believe that Allah sent many prophets before the final prophet Mohammad. [Note: "Allah" is just the Arabic word for God, used even by Christians in Lebanon and other countries. It is probably etymologically related to Elohim, the Hebrew word translated as `God' in the Bible. There are important theological differences between Islam and Christianity, but the name used for God is not one of them!] God has no sons or daughters. Instead Adam, Abraham, Moses, David, Mary, and Jesus were all prophets, speaking a message from Heaven, and their religions were all essentially identical to Islam (allowing for minor differences in ritual observance to suit different times and places), but unfortunately their teachings were corrupted over time. According to Islam, Jesus is merely the second-most important prophet in history; he did not did not claim to be the Son of God or divine:
When God says: "Jesus son of Mary, did you say to people, `Take me and my mother as two gods alongside God'?" he will say, "May you be exalted! I would never say what I had no right to say—if I had said any such thing you would have known it: You know all that is within me, though I do not know all that is within you, you alone have full knowledge of things unseen—I told them only what you commanded me to: "Worship God, my Lord and your Lord." I was a witness over them during my time among them. (Haleem translation of Sura 5:116-117)
The Quran also denies that Jesus was crucified or died, explaining that it only seemed like that to the bystanders:
“They did not kill him, nor did they crucify him, though it was made to appear like that to them; those that disagreed about him are full of doubt, with no knowledge to follow, only supposition; they certainly did not kill him—No! God raised him up to himself.” (Haleem, Sura 4:157-158)
And therefore the Quran also implicitly denies that Jesus has been resurrected from the dead, although Muslims do believe that Jesus was taken into Heaven, and will return to rule over a Muslim kingdom during the End Times.
I find these claims of continuity with the past to be significantly less plausible. It is true that Islam's strict Monotheism is superficially more similar to the teaching of Judaism, but it makes far less use of biblical prophecies and themes. And the fact that Muslims have to resort to saying that the Torah and New Testament became corrupt whenever they contradict Islam, shows that they do not in fact fit very tightly to Islam. It is very implausible that the process of copying the New Testament could have resulted in such thoroughly profound changes to the text as would be required to make the originals compatible with Islamic belief. Thus, a sensible Muslim must claim that our earliest written Christian texts, even those purportedly written by Jesus' own disciples, radically misunderstood Jesus' own teachings.
The claim that Jesus was never crucified is especially historically implausible, since this is a fact which virtually every serious scholar (outside the Muslim world), even those generally skeptical of the historicity of the New Testament, agrees is historically certain. After all, nearly all of the early sources that mention Jesus, including those by non-Christian Roman historians, allude to his execution. (One text that may date as far back as the 6th century, the Gospel of Barnabas, claims that Judas was substituted for Jesus on the cross, but this work is highly anachronistic and has no real historical value.)
Of course most of the early texts that mention Jesus' death also mention his Resurrection, which is not generally accepted by non-Christian scholars, because then they'd end up being Christians. We will examine the evidence for the Resurrection later, but for now I'll note that there are solid reasons why even those who doubt the resurrection accounts should still accept that Jesus was crucified. Crucifixion was an extremely shameful way to die in the Roman world, and it is quite unlikely that Christians would have invented a story their Master died this way if it hadn't actually happened.
There is a natural tendency to resist the idea that a divinely appointed and honored prophet could suffer such an ignominious fate. Yet in attempting to save the Christ from dishonor, Islam implicitly denies the most powerful ethical themes in Christianity: love, sacrifice, and the triumph of God even in weakness and suffering. From the Christian perspective, this is as if one bought a generic-brand pill from a pharmacist containing everything except the active ingredient of the name-brand pill. The active ingredient is God suffering on behalf of human beings. Without this, there is no Christianity.
I admit, of course, that an omnipotent Deity would have the ability to successfully hoodwink everyone about what happened to Jesus. But it is very hard to see a good reason why he would perform this particular trick. First of all it would raise serious questions about God's truthfulness, which is fatal in a religion based on divine revelation, which requires trust in God's veracity. I could perhaps see a motive to fool the enemies of a true prophet, but what motivation could there be to trick Jesus' faithful original disciples, whom the Quran speaks positively of:
O you who have believed, be supporters of Allah, as when Jesus, the son of Mary, said to the disciples, "Who are my supporters for Allah ?" The disciples said, "We are supporters of Allah." And a faction of the Children of Israel believed and a faction disbelieved. So We supported those who believed against their enemy, and they became dominant. (Sura 61:14)
Now consider the fact that the actual historical effects of Jesus' apparent Crucifixion—and, although the Quran does not mention this explicitly, his subsequent appearances to his disciples—were to convince his loyal disciples of (what Islam considers to be) false and idolatrous theological claims: namely the divinity of Jesus, and his ability to atone for the sins of the whole world through his death and Resurrection. Given the verse I just quoted, it doesn't really seem consistent to blame the disciples for the mixup. So it seems like God would be responsible for starting Christianity off on the wrong foot.
It's one thing for God to allow false religions to arise through the mendacity of charlatans; it's quite another for him to create them himself by means of direct and misleading miracles. If God does such things, it would throw into doubt the claims of any religion based on divine revelation. How would Muslims know that the miracles done for Mohammad were not equally deceptive? One could never know for sure.
Bible Characters in the Quran
A related topic worth mentioning is the large number of stories about Bible characters found in the Quran. Sometimes (as in the sura about Joseph), the quranic story is quite similar to the biblical narrative. In other cases (such as the depiction of Ishmael) things seem distorted and inconsistent. Now since the Quran is about 2,000 years farther removed from the people in question, it is a little difficult to believe that the traditional Arabic stories about these people were more reliable than the Hebrew traditions that went into the Torah.
The obvious reply for a Muslim, is to say that since the Quran is the direct word of God, the stories are not in fact based on Arab legends; instead they are divinely revealed accounts of what really happened! Thus these stories were not transmitted to the present day by the usual processes governing history and myths; instead they were directly revealed by supernatural means. But the trouble with this hypothesis, is that it fails to explain why the Quranic stories are so similar to the obviously legendary traditions that were already circulating at the time of Mohammad.
A very clear test case is the suras about Solomon. In the Bible (e.g. 1 Kings 1-11), Solomon is portrayed as an ordinary human being, very wise and rich, but not endowed with magical abilities. Apart from a couple visions in dreams, and a single supernatural sign to confirm God's acceptance of the Temple, nothing happens in the account of his reign which is not naturally possible. But the Solomon of later Jewish legend is a veritable magician! He can have conversations with birds and animals, and he can also enslave jinn (i.e. genies) with mighty magical spells and bindings, in order to force them to do his bidding, for example to help construct the Temple. (Even though sorcery was forbidden to pious Jews.)
Now to me there is no serious question which of these is the historical version, and which is the fantasy-novel version. But the Solomon of the Quran is the mythological Solomon. For example, consider this excerpt from the Sura of the Ant:
Solomon was David's heir. He said, "O people, we have been endowed with understanding the language of the birds, and all kinds of things have been bestowed upon us. This is indeed a real blessing." Mobilized in the service of Solomon were his obedient soldiers of jinns and humans, as well as the birds; all at his disposal.
When they approached the valley of the ants, one ant said, "O you ants, go into your homes, lest you get crushed by Solomon and his soldiers, without perceiving." He smiled and laughed at her statement, and said, "My Lord, direct me to be appreciative of the blessings You have bestowed upon me and my parents, and to do the righteous works that please You. Admit me by Your mercy into the company of Your righteous servants."
He inspected the birds, and noted: "Why do I not see the hoopoe? Why is he missing? I will punish him severely or sacrifice him, unless he gives me a good excuse." He did not wait for long. [The hoopoe] said, "I have news that you do not have. I brought to you from Sheba, some important information. (27:16-22)
To say that this is historical, would require believing that for some reason the Jewish chroniclers excluded from their own history all of the supernatural accomplishments of their greatest king; and then (over time) the stories about Solomon became gradually more and more accurate; until finally, under divine inspiration, it was revealed that these later myths were basically the same as what had originally happened. Even leaving aside the absurdity of the tales themselves (if taken literally, rather than as fables), that is not how oral tradition works.
Now, let me be clear: I am not a fundamentalist when I interpet my own Scriptures. (If we interpret either the Bible or the Quran as teaching that the Earth was literally created in just six days, then we have much bigger problems then Solomon talking to animals!) So if a Muslim wanted to say: God was simply using fictional stories believed in by the surrounding Arab culture to teach important spiritual lessons, then in principle I would not have any objection to that method of teaching.
But most Muslim theologians seem to have a much stricter standard for the literal historicity of the Quran than that. Their view of Scripture is not quite the same as that of the Bible. Jews and Christians claim that the Bible is a product of divine inspiration working through the instrument of human writers, but Muslims believe that the Quran is the words of Allah revealed verbatim from heaven by the angel Gabriel, with no admixture of human skill or opinion. I think that few Muslim theologians would consider giving up the literal accuracy of the stories about Bible characters.
And if they did, such an admission could potentially undermine the quranic claim to present a more accurate description of Jesus, whose nature is the key point of dispute beween Muslims and Christians. (Indeed, some of the quranic stories about Jesus can also be identified as dubious Christian legends: for example the story of Jesus making a clay bird come to life comes from the 2nd century Infancy Gospel of Thomas; which fills in the gaps of Jesus' childhood with various capricious miracles, as if he were a superhero who hadn't yet learned to control his powers.)
There are several more recent religions, such as the Baha'i faith, which claim in turn to fulfil Islam. But if Islam is untenable, then it seems any religion based on it must also fail. Or, even if Islam were correct, the fact that Mohammad seemingly claimed to be the final prophet would still invalidate them. So there seems to be no way forwards in that direction.
In the Gospels, Jesus explicitly warns that many false prophets would come in his name afterwards, and even lead people astray with deceptive signs and wonders. This seems like something to take into consideration, when evaluating any religion that claims to extend his teachings.
Moving over to the East, Hinduism is also an ancient religion (or perhaps more accurately, an eclectic mix of many different religious traditions, some of which are ancient). This adds somewhat to its credibility, for reasons similar to our discussion of Judaism. (If many people have been saying the same religious ideas for a long time, those ideas seem somewhat more likely to be based in objective reality than if one person has a revolutionary new idea about the gods, that no one has ever thought of before!)
On the other hand, Hinduism is considerably less unique looking than Judaism. At the popular level the religion looks like standard paganism (i.e. polytheistic idol worship), but then there is an overlay of various more sophisticated philosophies, that teach more unified systems such as Pantheism, Monotheism, Materialism, Dualism, Nihilism etc. This is more or less what the ancient Roman Empire looked like before Christianity became dominant there. Of course, that does not automatically imply that any of these philosophical schools are wrong (that would require going into specifics) but it looks much less like a definitive supernatural revelation from Heaven, and a lot more like what you would expect people with a strong religious instinct to come up with on their own.
Buddhism clearly took over a number of important ideas from Hinduism, most notably the goal of achieving liberation from the cycle of rebirth. The main reasons why Buddhism is regarded as a separate religion, rather than as a branch of orthodox Hinduism, are that 1) it does not accept the Vedas of Hinduism as religious scripture, and 2) it subscribes to the doctrine of anatman ("No Self") which denies that human beings have any permanent essence to subscribe to. In these respects, Buddhism is a novel religion making a fresh start; it does not claim to "fulfil" Hindu religious scriptures the way Christ claimed to fulfil the Jewish scriptures.
On the other hand, I was surprised to learn a while ago that Buddha is regarded by many Hindus as the most recent Incarnation of Vishnu, sent "to deliver the pious and to annihilate the miscreants, as well as to reestablish the principles of righteousness." In fact he is the only Incarnation on their list to appear in historical times (i.e. after writing came to India, but before the future). And indeed, when I visited India to teach at a summer school, we went on a day-trip to the Konark Sun Temple and saw a relief carving of Buddha on this Hindu religious site.
But if that's true, then I don't really understand why these Vishnuists don't convert to Buddhism, since apparently that's the most recently updated version of the religion! (When it comes to software updates, I'm not much of an early adopter. I'd rather stick with what works, rather than migrate immediately to the newest version. But if the next version of Windows came with a recommendation by God himself, then I might think differently about it. And a couple dozen centuries is surely long enough to check that the new version is stable!)
Some Eastern religions, such as Sikhism, incorporate elements from both Islam and Hinduism. This is a pretty audacious feat, since it is hard to imagine two religions that are, considered in themselves, more incompatible! However, Sikhs identify as neither Muslim nor Hindu, and consider themselves to be an entirely new monotheistic religion, even though their Scripture includes the writings of certain Hindus and Muslims.
What happens to outsiders?
One important question that arises in any religion that claims to be revealed at a particular time and place, is what to say about those from other cultures who were never influenced by the message? This is less of a problem for inclusivist religions (that teach that salvation can be achieved in any religion) or religions that teach Reincarnation (since these people could say that everyone will eventually be born into the right culture). But in the case of Christianity and Islam, it raises more serious difficulties, since these religions teach that everyone ought to convert to the true religion.
(Indeed the problem is even more severe for Christianity, which teaches that union with Jesus is the actual cause of salvation. Islam does not teach that Mohammad is a Savior; he only brought the final and most complete message telling people what God expects them to do. This is similar to Pelagian heresy condemned by orthodox Christianity, the belief that human beings are already capable of pleasing God naturally, and only need instruction in how to behave, not a radical transformation of the human condition.)
In either case, I think the right strategy is to appeal to God's justice and mercy, to argue that people will be judged on the basis what they knew, even if they were not part of the truest religion. I personally think the best interpretation of the Bible allows for the possibility of salvation for those who die as non-Christians, although like Christians they would be saved only through the sacrifice of Jesus.
If I were a Muslim, I would adopt a similar interpretation of Islam. Even though there are passages suggesting that disbelievers go to Hell, there are also passages that explicitly allow for the possibility of monotheistic "People of the Book" (such as Jews and Christians) to be judged as righteous. (On the other hand, orthodox Christians who consider Jesus to be divine would be committing shirk, the worst sin in Islam.) It is also stated that God does not punish anyone unless he first sends a messenger to warn them:
Whoever is guided is only guided for [the benefit of] his soul. And whoever errs only errs against it. And no bearer of burdens will bear the burden of another. And never would We punish until We sent a messenger. (Sura 17:15)
suggesting that even some pagan idolaters might end up being saved, if they didn't know any better.
Since the foundational principle for both religions is God's goodness, it follows that we can trust God not to judge people unfairly. Given that there is a life after this one, God's choice to reveal his truth by a historical process (initiated at particular times and places) does not require him to leave everyone else out in the cold permanently.
Next: Supernatural Claims
[4/6/19 Updated to add section headers, fuller discussion of fulfilment, and more minor corrections]