The oldest manuscripts of ancient writers like Aristotle, Plato, Herodotus (among other) amounts to a small number of copies that were made a thousand years or more after the originals were written. There are no more then ten manuscripts of Julius Caesar's Gallic Wars, and the oldest copy of that was written over 900 years later than the original. Scholars accept these documents as adequate representations of the originals.
Why not the Bible?
The earliest portions of The New Testament date to within just 25 years of the originals. Some nearly complete books of the new testament date to within one century or less from the originals. And we're not even talking about a handful of copies that can be compared with one another to determine accuracy or consistence. There are nearly 25,000 complete manuscripts of the New Testament, with more than 15,000 that date to before the 7th Century A.D. (or C.E. if you prefer). These include 5,300 copies in the original Greek, over 10,000 in Latin Vulgate, 4,100 Slavic translations, 2,000 Ethiopian translations and about 1,000 other early translations.
Further, in the first centuries after Christ, thousands of letters, and other documents were written in which people quoted from other documents that would later be assembled into what was to become the New Testament.. These quotes are so extensive that even if there wasn't a single Bible in existence, you could go back to those letters and documents and using only those written within 250 years after the death of Christ, you could find every word of the New Testament, with the exception of 11 verses.
There are small differences in all those manuscripts - however, all these differences, most are a matter of spelling or word order changes that were made as the styles changed over the ages. In fact a total of only about 200 words, or 1/10 of 1 percent of the entire new testament are subject to more than trivial differences. And no single doctrine of Christianity in all it's denominations throughout history depend on a piece of disputed text.
As for the Old Testament, the discovery of the Dead Sea Scrolls show that in over 2,000 years those who copied the Old testament were so meticulous that no significant changes were made to the texts. The Dead Sea Scrolls represent a major library of over 800 total documents dating between 250 B.C. to 68 A.D. Every book of the Old Testament is included except for some minor prophets, and Esther.
