Maybe Jesus did not exist in a purely historical sense, but we have to consider one small thing here, too: Has any character in any book of fiction ever been entirely 'original?' Probably not - Characters in any book, if they are to be believeable in some way, make people relate to them, have to be based on actual people who do exist or have existed.

I don't doubt that much - Wether or not the Jesus of the Bible lived, as a single person, and wether or not the apostles depict his life history accuately, what I think I can safely say is that assuming he did not really exist, he was still inspired by people who did.