The question is then how can we decide what anybody in the ancient world said. We can’t. We wish we could. It would be nice if we could. You would like to think that because you can go to the store and buy an edition of Plato that you are actually reading Plato, but the problem is that we just do not have the kind of evidence that we need in order to establish what ancient authors actually wrote. In some cases, we have all these data, and sometimes we have just one manuscript. Sometimes we have a manuscript that was written two-thousand years later, and that’s it! So, as much as we would like to be able to say we know what ancient authors actually wrote, we often just do not know.

Bart D. Ehrman (b. 1955) American Biblical scholar, author

“The Textual Reliability of the New Testament: A Dialogue between Bart Ehrman and Daniel Wallace,” Greer-Heard Point-Counterpoint Forum in Faith and Culture (2008-04-04/05)
    (Source)

Reprinted in Robert Stewart, ed., The Reliability of the New Testament (2011).