Jesus was a real historical figure. His being fictional just doesn't make sense -- same way that Mohammed being fictional doesn't make sense -- or do you also believe Mohammed fictional?
"There is more evidence for Jesus than X" turns out not to be such a good argument either.
And there is serious historical thought that Mohammed, too, was fictional. The Wikipedia article maps out the scanty evidence.
The real problem with either question is the excessive interest in getting the "right" answer. If e.g. Socrates turned out to be a fictional character invented by Plato, philosophy wouldn't care. If Gautama Buddha turned out to be fictional, Buddhism wouldn't care. But Jesus or Mohammed existing or not is a REALLY BIG DEAL, and we really don't have a great deal of evidence in either case. Considerably less for Jesus, and a lack of evidence where it would have been expected had he existed.
And there is serious historical thought that Mohammed, too, was fictional
Yeah, that's a ludicrous idea too. Some people seem to think that "fictional" is the null-hypothesis, to be believed by default unless there's an extraordinary amount of evidence to the contrary. This is nonsense: Fictional characters of historical influence aren't more common than real people of historical influence.
This is bias at its most obvious.
If you believe that someone made a fictional character up, and then he made thousands of near-contemporaries believe in his...
To break up the awkward silence at the start of a recent Overcoming Bias meetup, I asked everyone present to tell their rationalist origin story - a key event or fact that played a role in their first beginning to aspire to rationality. This worked surprisingly well (and I would recommend it for future meetups).
I think I've already told enough of my own origin story on Overcoming Bias: how I was digging in my parents' yard as a kid and found a tarnished silver amulet inscribed with Bayes's Theorem, and how I wore it to bed that night and dreamed of a woman in white, holding an ancient leather-bound book called Judgment Under Uncertainty: Heuristics and Biases (eds. D. Kahneman, P. Slovic, and A. Tversky, 1982)... but there's no need to go into that again.
So, seriously... how did you originally go down that road?
Added: For some odd reason, many of the commenters here seem to have had a single experience in common - namely, at some point, encountering Overcoming Bias... But I'm especially interested in what it takes to get the transition started - crossing the first divide. This would be very valuable knowledge if it can be generalized. If that did happen at OB, please try to specify what was the crucial "Aha!" insight (down to the specific post if possible).