[Diamond]

Or that he didn't say it at all, and the words were put in his mouth by the author.

Why do all the anti-Es conveniently forgot that particular option?

For the same reason, except to a degree many orders of magnitude higher, that no one questions the authenticity of the words of Plato, Aristotle or Homer, et al.

Cordially,

[Junior]

We have third-party references for the Greek guys. All we have are the Gospels for Jesus. We don't even have any Roman records of his existence, and the Romans kept track of everything -- especially folks who caused enough trouble to be crucified.

[ToryHeartland]

no one questions the authenticity of the words of Plato, Aristotle or Homer, et al.

Let me modestly suggest that there is in fact a centuries-old industry dedicated to precisely this task; it is called classical philology, and it is the basis for much of our understanding of the ancient world.

[Lurking Libertarian]

that no one questions the authenticity of the words of Plato, Aristotle or Homer, et al.

I don't know anyone who accepts the truth of Homer's religious statements, or the historical accuracy of his accounts.