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.
1. I don't think manuscript copying errors are anywhere near the top of any reasonable list of reasons for skepticism about the accuracy of the New Testament.
2. There is another way for errors to survive undetectably without either preceding the first extant branch or requiring a giant political controversy: an especially likely error -- e.g., correcting something that looks like an obvious mistake -- could happen independently on both sides of the branch.
I doubt there are many cases of this, and I wouldn't be surprised if the number were zero. But I think it should be said for completeness' sake.
3. While it's true that "we have thousands of different manuscripts before the printing press, some of which are quite early, starting with fragments in the early 2nd century", there's a lot of positive spin in that statement. Here is the complete list of early second-century NT manuscripts: P52, containing parts of five verses of John (about 25% of each), c.125. That's it. Oh, and that tiny fragment that's the earliest thing we have from the NT? It contains a copying error; more precisely, it contains strong evidence of a copying error in the now-missing bits between its words. (There isn't enough space to fit the correct text into one of the gaps.)
There are a bunch of other second-century manuscripts. Maybe six. (The exact number is unknown because the datings are unknown; a lot of the candidates are best described as "c.200".)
There are indeed thousands of manuscripts that predate the printing press -- but they're almost all many centuries later than the originals. Nice to have, for sure, but having thousands of them isn't actually much more use than having, say, a hundred.
So, anyway, the point is that this doesn't really justify saying "the first branch point has to have been incredibly early". Earlier than for many other ancient documents, for sure, but the right conclusion is more "so much the worse for other ancient documents" than "so much the better for the New Testament".
4. Why would you think the Textus Receptus is 50 generations of copies removed from the originals? The TR is itself the result of textual criticism, albeit on a much thinner set of documents than we have now, so what matters is (roughly) the dating of the TR's branch points; and I see no reason to think that those are anywhere near 50 generations from the originals. I suggest that 10 generations would be a much better estimate; or, to put it differently, perhaps the TR is about 3x worse than the text we have now.
That would suggest that perhaps 40 verses of the text we have now contain major errors. (I should say that feels like an overestimate to me.)
5. The simple model that says errors occur uniformly and independently at random in the NT is pretty dubious. There may be more errors in texts that copiers were more strongly motivated to change. This weakens the force of your point about doctrines being found in multiple places; if some particular verse is wrong, then other similar verses are more likely also to be wrong.
6. I repeat that I don't actually think copying errors are a large contributor to the reasons we have for not believing everything in the New Testament. They're just a rather thin layer of icing on the cake.
I agree with pretty much everything g says. I doubt we've lost much of anything we hadn't already lost by 150-200 AD.
I'm not sure if the copy error rate before (say) 150 AD ought to be assumed to be the same as the later one. I can see arguments for higher error rates (people knew they wouldn't be caught) or lower error rates (people tried harder to get it right since they knew the texts were rare). But this is a very minor point.
I expect most of the distortion entered the account during the ~40 years when it was an oral tradition, and in the period immediately after as the first books were being distributed and the second books were being written.
Incidentally, textual criticism gives us a feel for what types of errors scribes make. The category of error that produced I John 5:7 is alarming-- it means that scribes occasionally insert a point of critical doctrine into the text. If the folks who actually first wrote the books were not above this... Well, I guess this is an argument that the errors present in the first generation or two of manuscript were probably not independent.
lavalamp & g,
If I'd included every possible caveat, this post would have had to be several thousand words longer, and it was enough work as it is! I'm trying to write for a popular audience here. I know that this issue doesn't play an important role in your skepticism, but I felt I had to include it for the sake of others.
I don't think I put excessive positive spin into my statement: "fragments" is not exactly a reassuring word, now is it? One should also be interested in at what point high-quality complete manuscripts become available, and I think the answer here is the 4th century.
The mere existence of numerous late copies doesn't necessarily put the first branch point early, but we have good reasons to believe the documents were widely distributed early on. After all, they were being read liturgically in Christian worship services almost as early as we have post-NT evidence for Christian worship services.
When I said that the Textus Receptus was 50 generations removed, this was just dimensional analysis---an estimate to within an order of magnitude or so. You're right that the Textus Receptus is itself the result of textual criticism, as was its predecessor the Byzantine text (the ancients weren't total dummies about this sort of thing). However, the Textus Receptus was created using just 6 late Greek manuscripts, all but 1 in the Byzantine tradition. If the resulting text was the majority reading of these, the effective branch point was probably quite late. Unfortunately ancient attempts at textual criticism make modern attempts more difficult, but the wide geographical distribution of the manuscripts means we don't have to rely on the Byzantine texts.
Of course the Johannine Comma is disturbing, but that's pretty much the worst of the examples (besides the two long sections); most of them aren't like that. And it probably wouldn't have happened if the doctrine of the Trinity weren't already commonly accepted due to other parts of the NT. What would be really disturbing is the insertion of a major doctrine not contained in the rest of the NT, and I don't know of any instances of this.
Agree with nearly all of that save, "And it probably wouldn't have happened if the doctrine of the Trinity weren't already commonly accepted due to other parts of the NT."
I'm not an expert, so I won't take a firm position, but to me it seems at least plausible that it was added precisely because the Trinity was disputed and lacked clear support in the rest of NT.
(I'd even go so far to say that even if the Johannine Comma were legit, it's not clear to me that the orthodox christian view on the Trinity is anything but an accident of history. The position in the Athenasian Creed is one possible harmonization of the NT/OT, but probably not the only one. I say this just to dispute the view that the doctrine of the Trinity is clear in the NT.)
Well, it depends on what you mean by the doctrine of the Trinity. The precise details were hashed out over several centuries, but I think that what one might call "Triadism" is quite clear in the NT. By this I mean the belief that there is a theologically important set with three members, named 1. Father/God/Lord, 2. Son/Jesus/Christ/Lord, 3. (Holy) Spirit (of #1 or #2), which at the very least describe various aspects or mediators of God's relationship to us, and which are invoked together in certain acts of Christian worship. I think this would have been common ground between the various later factions, the question being whether Triadism implies the full divinity of the Second and Third Persons--if it does, then Monotheism requires one to formulate something not unlike the doctrine of the Trinity.
While the Johannine Comma might be helpful in going from Triadism to the Trinity, it doesn't actually add all that much which isn't already contained in e.g. Matthew 28:19, John 10:30, and the other classic proof texts. In other words, an Arian who wished to deny the divinity of Christ could easily apply the same kind of interpretations to the Comma that they would use for other passages. I think it's more likely that someone was carried away by 1 John's mention of a set with three members, one of which was the Holy Spirit. This would pretty much inevitably cause Christians to think of the Trinity, although a second glance should have indicated that conjunction of the Spirit with two inanimate substances isn't really primarily about the Trinity.
This isn't really on-topic, but I'd like to say as a response to the other thread, that the Athanasian view of the Trinity really shouldn't be viewed as a direct logical contradiction in terms. After all, it's only a logical contradiction if God is one and three in the same respect, whereas the standard language is that the category which God is one of (substantia/ousia) is different than the category which he is three of (persona/hypostasis). That, plus the recognition (famously expressed by St. Bill Clinton) that "is" is an ambiguous word, is quite sufficient to make the Trinity consistent as a matter of formal predicate logic, even if it seems weird to our puny minds. But you must understand this, or else you would have assinged Trinitarian Christianity a probability of 0.
Yes, you're right, essentially all of my probability mass for Christianity is in the "Christians don't have the trinity quite correct" category. I view the Trinity as a particular harmonization of the facts you mention in your first paragraph, which was competing with other harmonizations (and ultimately won).
I understand that there is supposed to be a substance/person distinction, but if you read the Athanasian creed, it seems to painstakingly go through a bunch of attributes, and for each one it says each person has it but only one thing has it. In particular, my understanding is that the view that each of the persons is an aspect of god is heretical. Each person is supposed to be the complete, entire god. My probability that this is even a coherent position is pretty low. The Athanasian creed is substantially more specific than the NT, so I'm sure there are other ways of harmonizing things. (E.g., the Aryan heresy, although that's probably a worse thing to be heretical about, as far as the average christian is concerned.)
(BTW, I don't give logical impossibilities exactly 0 probability, because there is some chance I could be mistaken about the logical impossibleness of it!)
Well, I would rather interpret things like the (so-called) Athanasian Creed in such a way that it is logically consistent. I don't think this places me entirely outside the Christain tradition. In any case, the Nicene creed is closer to being a consensus document for all Christians, and I don't think it suffers the same degree of logical difficulties, although some of the key terms (e.g. "begotten") are obviously metaphorical even on the orthodox view.
If by "aspect" you mean just aspects of the way God relates to us, which don't actually correspond to any eternal distinction in God's own nature, then the position is indeed regarded as heretical. On the other hand, if it means aspects of God's nature which are distinct even for God, then I don't think it's heretical. I didn't mean the word "aspect" to imply a particular position on this point.
I think--if I were still a christian--I'd still have some nits to pick with that, but given that I'm not, I don't have much to say. :)