An historian has written a book arguing that Jesus never existed. His hypothesis on how the christian religion developed is something I will not go into here, but I want to address a question he raises in the interview linked, namely, why is it that atheists tend to fairly adamantly maintain that there was an historical man around whom the religion grew?
I hypothesize two motivations, which certainly apply to me, and which likely apply to many other atheists.
The first reason is our noble skepticism. While we are skeptical of the claims of miracles, we are also skeptical of the claim that a religion could grow up centered around an individual person within a few decades of when that person lived without that person existing at all. It is not impossible, but it seems relatively unlikely.
The second reason is much less intellectually honest: it is about being persuasive to believers. I think there is a feeling, perhaps not even completely conscious, that if we say to a believer ‘Jesus probably didn’t even exist,’ that they will not take us seriously, whereas if we say ‘Jesus was probably an ordinary teacher around whom a mythology developed,’ they are more likely to listen to our arguments.
This is, of course, a very bad reason to hold a position. We should hold positions based on evidence, not because we think others will be more likely to agree with us or take us seriously.
Nonetheless, I find the hypothesis that Jesus was completely made up less persuasive than the hypothesis that there was an historical guy whose life was mythologized.