A lot of people think that the New Testament documents must be unreliable because (like all published texts from the ancient world) they are based on translations of copies of copies. So how can we know whether the original version of the text is the same as what's printed in our Bibles? Isn't it like a game of telephone, where eventually the message becomes distorted beyond recognition?
This issue came up in the comments section of the previous article, but I think it is important enough to justify its own post, especially since there are interesting connections to evolutionary biology. (Yes, really!)
First let's consider translations. All modern translations of the Bible are based entirely on direct translation from the original Greek (for the New Testament) or Hebrew (for the Old Testament). They are not translations of translations (except insofar as the original texts may already be translations of e.g. Jesus' words from Aramaic into Greek). Now I won't deny that the translation into English may obscure or distort features of the original language. As the Italian proverb has it, traduttore, traditore. However, this problem is mitigated by the fact that there exist lots of different scholarly translations of the Bible with different agendas, and we can check them against each other. Or, if we are really dedicated, learn ancient Greek or Hebrew ourselves. So let's set aside the translation issue, and just consider the question of whether the Greek text used by the translators is the same as the original Greek text.
In fact, we can be extremely confident that—with a couple important exceptions that I'll discuss below—the Greek New Testament used to make your Bible is in almost all important details the same as in the orignal text penned by the author (or more likely, dictated to a scribe). And, in cases where the text isn't certain, we almost always know it isn't certain, and a scholarly edition of the Bible will point out the possible variants in a footnote.
So how do we know this? The answer is using a field of study called "textual criticism", a procedure which is sufficiently methodical and precise that it almost qualifies as Science rather than History. The idea is that manuscript copies don't just form a straight line, they form a tree with branches. That's because a single manuscript may be copied more than one time, and hence have multiple descendents. The root of the tree is the original manuscript (the autograph), and the branches are the various families of manuscripts which descend from it.
It's a lot like the "Tree of Life" in evolutionary biology. The different species we see can be classified into a tree structure based on how recently they diverged from their most recent common-ancestor species. By comparing the DNA of different species, you can identify at what points in history various specific mutations must have arisen (e.g. if the mutation affects chimps and humans but not bonobos, it must have occured after the proto-bonobos split off, but before proto-chimps and proto-humans went their separate ways).
Well, it's the same with manuscripts of the New Testament. From time to time, copying errors were introduced, either accidentally or on purpose. These errors propagated into the copies of that manuscript, but unrelated manuscripts were unaffected. What that means, is that the only way an error could go undetected, is if it occured before the first branch point such that we have surviving manuscripts for both branches. (Or if there was a giant political conspiracy to change every copy of the New Testament in existence, but there's no evidence of any such thing, and once the Chrisitan church had enough centralized political power, the manuscripts were circulated far too widely for this to be feasible.)
If the error occurs after the first branch point, we'll have different manuscripts which say different things in some place. It's a separate and harder question to figure out which of these variants was the original. One could just count them (the "majority text") but this might not work since the branch with the error could have proliferated more widely than the others. Instead, most scholars try to reconstruct the family tree, and weight more heavily manuscripts which don't seem to be closely related to each other (the "critical text"). However, since these variants usually don't affect major points of Theology, mostly I only care that we can identify the passages where there might be a discrepency—in these cases, pick whichever variant is your favorite, and I'll argue based on that edition of the New Testament.
In the case of the New Testament documents, the first branch point has to have been incredibly early. That's because the New Testament was extremely popular: we have thousands of different manuscripts before the printing press, some of which are quite early, starting with fragments in the early 2nd century, and going onwards from there. The runner-up is Homer, who was also well-liked, and therefore we have a few hundred copies. By comparison, for most other ancient Greek and Roman texts (e.g. Aristotle) we're happy if we can find a few dozen copies from the high medieval ages. The results of textual criticism are usually regarded as reliable in these cases; how much more so when the available "fossil record" is so much more detailed!
Let's try to quantify the probabilities a bit more closely. I'm not a professional textual critic, so I'm only going to attempt a rough back-of-the-envolope estimate here.
The only way that an error could sneak in unnoticed is if it occured before the first branch point. Consider as an example St. Paul's letter to the Galatians (which nearly all scholars think was written by Paul himself). Let us define the "Publisher" of Galatians as the first person who arranged the text to be copied many times, where "many" means enough times to ensure that at least two copies have surviving descendents. In the case of a famous person such as Paul, whose writings would be in high demand in many churches, it seems likely sufficient to copy the manuscript a mere 5 or 6 times, and then send them to geographically diverse regions in the Roman empire. Now, the most likely person to be the Publisher is Paul himself. However, we cannot rule out that the autograph was copied once or twice, maybe even three times, before the Publication. (More than this seems unlikely to me; why would people be interested enough in a text to copy it once, several times, but not interested enough to distribute multiple copies?)
To estimate the rate at which copying produces errors, let's consider the Textus Receptus (the line of descent which gave us the King James Version), circa 1500. Let's suppose this is about 50 copies removed from the first branch point (that's about one copy per 30 years) There are about 8,000 verses in the New Testament. Of these, two large sections of the Gospels, the longer ending of Mark (12 verses) and the Woman caught in Adultery in John (12 verses), were probably not in the original manuscripts (although they are early enough that they may well date back to the first century). In addition to these, Wikipedia lists 100 verses (I counted, that's the exact number) with major discrepencies between the Textus Receptus and the accepted critical text. There would be a lot more if we counted minor variants, but since most of the "major differences" on this list don't significantly affect interpretation, I'll draw the line here.
Putting all this together, we arrive at a "major error rate" of about .0003 per verse per copy. If we allow for 3 copies before the first branch point, you still have to pay about in probability ratios for each (noncontiguous) verse of the New Testament you'd like to dynamite. Since the most important claims in the New Testament are typically found in several places, that's a very inefficient form of skepticism.
So the evidence suggests that a modern critical New Testament is at least about .999 accurate to the original text. To be clear, this doesn't tell us anything about whether the books were really written by their traditional authors, or whether the original claims are true; that's a subject for other posts. It does tell us, though, that the original claims were in fact made, by some first century person (whoever wrote the book in question), who was able to pass himself off to some audience, as a person with apostolic credentials.