Last time, I ended with a question: what are the Jews famous for?
The answer, of course, is Monotheism. The Jews are famous for either bringing into the world, or else preserving, the doctrine that there is just one God.
It's important to realize that this is not just a matter of counting, as though Monotheism were the golden mean between Polytheism and Atheism. It's not just about having the right number of gods. It's a matter of what kind of gods you believe in.
Polytheists believe in gods which were born at particular times and places, and have limited spheres of influence. In other words, their gods are just like you and me, except for being a lot more powerful. In Greek or Norse mythology, most of the gods don't seem to have a moral advantage over human beings either. They just get their way more often—except when they are quarreling with each other.
Monotheists, on the other hand, believe that God is the fundamental entity in existence, that he has no external limitations, that he is perfectly good and wise, that he is eternal, and that he created everything besides himself. This, obviously, is a completely different kind of entity than the polytheistic divinities. And, equally obviously, there cannot be more than one of these. Logic says that there cannot be two rival Deities, each of which created everything else, including the other one.
Thus on the Monotheistic conception there can be only one God, and following the usual grammatical convention of capitalizing titles like "President" or "Dad" which single out a particular entity, one normally capitalizes the G. But as a convenience, since in the rest of the post I will be talking about the Monotheistic God side by side with Polytheistic gods, in the rest of this post I will always use the captial letter to distinguish the two, so I don't have to keep using adjectives to keep them apart. (*)
A certain sort of Atheist likes to say, "What makes Jehovah any different than Zeus, Thor, Athena, or Allah? Once you realize why you disbelieve in any of those gods, you'll know why I disbelieve in yours." But this completely misses the point. Thor and Athena are polytheistic gods, and they can both exist at the same time without contradiction. Whereas "Allah" is not a different God from the God of the Jews or Christian, it's a name for the same concept of an ethical Ultimate Reality who created everything. Monotheistic religions may disagree about how we should think about God, about what he is like, but we do not disagree about which God exists, since the basic concept is similar enough. That's why Arabic speaking Christians also refer to God by the name Allah!
Monotheists criticize Polytheists, not because they have too many gods, but because to us they don't have any! They are idolators, worshipping things which do not deserve to be worshipped. Worship is an act of total submission and reverence, holding nothing back. The polytheistic gods are not worthy of that. They are mere creatures like us. Polytheism is a form of Atheism.
In fact, the concepts of God and "gods" are so distinct, that one could even believe in the existence of both! We Christians believe in the existence of many "supernatural" beings (angels and demons) who are much more powerful than we are. But we don't worship them, because they aren't God; like us they are limited creations. As St. Paul says:
We know that an idol is nothing at all in the world and that there is no God but one. For even if there are so-called gods, whether in heaven or on earth (as indeed there are many “gods” and many “lords”), yet for us there is but one God, the Father, from whom all things came and for whom we live; and there is but one Lord, Jesus Christ, through whom all things came and through whom we live. (1 Corinthians 8:4-6).
It's important not to get confused, as St. John is near the end of the Book of Revelation:
I, John, am the one who heard and saw these things. And when I had heard and seen them, I fell down to worship at the feet of the angel who had been showing them to me. But he said to me, “Do not do it! I am a fellow servant with you and with your brothers the prophets and with all who keep the words of this book. Worship God!” (Revelation 22:8-9)
The flip side of this coin is that even worshipping just one god doesn't necessary square you with the Monotheistic worldview. To make a silly example, suppose that the Internet became a sentient being and that some former Atheists started worshipping it as the most powerful and wise being in existence. It would still be a polytheistic-type "god", even though there was just one of it.
Zeus is a particularly interesting case. Most people are familiar with Greek mythology, in which Zeus is simply the most powerful of the many gods. But many educated Greeks knew full well that mythology was just lies made up by poets, and used "Zeus" as the name for the monotheistic-type God. One Greek text, a sky altas quoted by St. Paul in his sermon to the Athenians, illustrates this confusion well. Aratus' Phaenomena begins by invoking Zeus as though he were the Absolute Creator who dwells in all things:
From Zeus let us begin; him do we mortals never leave unnamed; full of Zeus are all the streets and all the market-places of men; full is the sea and the havens thereof; always we all have need of Zeus. For we are also his offspring; and he in his kindness unto men giveth favourable signs and wakeneth the people to work, reminding them of livelihood. He tells what time the soil is best for the labour of the ox and for the mattock, and what time the seasons are favourable both for the planting of trees and for casting all manner of seeds. For himself it was who set the signs in heaven, and marked out the constellations, and for the year devised what stars chiefly should give to men right signs of the seasons, to the end that all things might grow unfailingly. Wherefore him do men ever worship first and last.
and then in the middle inconsistently recounts the origin of Zeus as a deity who was born at a particular time and place, with the world already in existence:
If, indeed, the tale be true, from Crete they by the will of mighty Zeus entered up into heaven, for that when in olden days he played as a child in fragrant Dicton, near the hill of Ida, they set him in a cave and nurtured him for the space of a year, what time the Dictaean Curetes were deceiving Cronus.
We see here that Greek religion and Greek mythology were not the same thing. Even more striking is the Stoic philosopher Cleanthes' Hymn to Zeus, written in the 3rd century B.C. Go ahead, click on the link. I wager you'll be a shocked at how Christian his poem sounds, if you read "God" for "Zeus".
To Monotheists, it is blasphemous to think of God as being the same as any created thing. He is more superior to an archangel, than the archangel is to a slug. Neither angels, nor souls, nor the heavenly realms, not even the entire universe, have any intrinsic religious significance to us at all. We desire God alone. As one modern Christian song says:
We are a moment, You are forever
Lord of the Ages, God before time
We are a vapor, You are eternal
Love everlasting, reigning on high
Only one is Holy, only one is Good. He is the Supernatural. Everything else is a limited, finite being, subject to laws set by the Almighty. They are natural. We don't always know their laws, but that doesn't change the fact that they part of a definite Nature, which originates from, and is circumscribed by, God's will. God is the Unlimited, in whose bosom all of these limited things exist.
UPDATE: Edited post in response to some criticism. The paragraph marked with a (*) used to read: "Thus there can be only one God, and following the usual grammatical convention of capitalizing unique titles, we capitalize the G."