Any ancient document (such as the New Testament), is derived by analyzing the copies of hand-written manuscripts which are still in existence. The reliability of such a document, then, is a function of the number and age of the manuscripts still available to us. And based on these criteria, there is no writing from the ancient world which is as well-attested as the New Testament. In fact, there is not other document which even comes close.

Also, the dates of the New Testament manuscripts lend a great deal of credibility to the document. The earliest surviving fragment dates to about 130 AD, and other manuscripts date to the 3rd century. The earliest copy of The Iliad, on the other hand, was recorded some 500 years after the book was originally written and the gap for The History of Herodotus is 900 years!

Finally, in addition to demonstrating that the New Testament we now have is an accurate version of the original writings, it should be noted that the NT books were written and distributed during the 1st century, when many people were still alive who were eyewitnesses of the life of Christ. For Jesus' biographers to have falsely attributed miracles and other divine characteristics to him would have been foolish, as they could have easily been refuted (think of a modern author claiming that Harry D. Truman healed the blind!). In the final analysis, one is certainly free to dispute or ignore the teachings of the New Testament, but there is little room to doubt their authenticity.