There's a common misconception about how the Bible we read has been handed down to us. You have probably heard someone say that we can't trust what the Bible says because it was passed from one person to another, possibly verbally, and it got changed down through the years. It's a linear model like the "telephone game." But that's not how it works. We actually have many, many copies and pieces of the New Testament that date back very close to when the original manuscripts were written, and scholars then assemble these and compare them. The consistency is a remarkable testament to the care scribes took when making copies, and it's quite easy to spot errors and determine what the correction is.
Greg has a silly illustration that demonstrates the important fact of why we can trust the text of the Bible we are reading in 2006:
Let me illustrate how such a test can be made. It will help you to see how scholars can confidently reconstruct the text from existing manuscript copies even though the copies themselves have differences and are much older than the autograph (i.e., the original).
Pretend your Aunt Sally has a dream in which she learns the recipe for an elixir that would continuously maintain her youth. When she wakes up, she scribbles the directions on a scrap of paper, then runs into the kitchen to make up her first glass. In a few days her appearance is transformed. Sally is a picture of radiant youth because of her daily dose of what comes to be known as "Aunt Sally's Secret Sauce."
Sally is so excited she sends hand-written instructions to her three bridge partners (Aunt Sally is still in the technological dark ages--no photocopier) giving detailed instructions on how to make the sauce. They, in turn, make copies which each sends to ten of her own friends.
All is going well until one day Aunt Sally's pet schnauzer eats the original copy of the recipe. Sally is beside herself. In a panic she contacts her three friends who have mysteriously suffered similar mishaps. Their copies are gone, too, so the alarm goes out to their friends in attempt to recover the original wording.
They finally round up all the surviving hand-written copies, twenty-six in all. When they spread them out on the kitchen table, they immediately notice some differences. Twenty-three of the copies are exactly the same. One has a misspelled word, though, one has two phrases inverted ("mix then chop" instead of "chop then mix") and one includes an ingredient that none of the others has on its list.
Here is the critical question: Do you think Aunt Sally can accurately reconstruct her original recipe? Of course she could. The misspelled words can easily be corrected, the single inverted phrase can be repaired, and the extra ingredient can be ignored.
Even with more numerous or more diverse variations, the original can still be reconstructed with a high level of confidence given the right textual evidence. The misspellings would be obvious errors, the inversions would stand out and easily be restored, and the conclusion drawn that it's more plausible that one word or sentence be accidentally added to a single copy than omitted from many.
This, in simplified form, is how the science of textual criticism works. Textual critics are academics who reconstruct a missing original from existing manuscripts that are generations removed from the autograph. According to New Testament scholar F.F. Bruce, "Its object [is] to determine as exactly as possible from the available evidence the original words of the documents in question."
The science of textual criticism is used to test all documents of antiquity--not just religious texts--including historical and literary writings. It's not a theological enterprise based on haphazard hopes and guesses; it's a linguistic exercise that follows a set of established rules. Textual criticism allows an alert critic to determine the extent of possible corruption of any work.