That's what making it the default position means, no? Because lack of enough evidence (according to you) that he was real, you treat his fictionality as a default position, even though you don't have any evidence in favour of it -- no evidence in favour of the existence of that supposed "cult leader" who authored him; and yet somehow was completely unknown to history.
Perhaps you can explain where your reasoning differs from mine using the vaccination/autism example? Absence of evidence is not evidence of absence, but absence of evidence after a search that should have found evidence if there was any evidence to be found is evidence of absence.
I don't have a default position that there is nobody on the footpath outside my house. However if I look outside at the footpath and I can see that there is nobody there, then that is strong evidence in favour of the hypothesis that there is nobody there.
Regarding your other arguments, I do not find the argument that the Jesus story is exceptional because it has elements X, Y, Z etc. not found in other messianic stories persuasive because it is a highly general argument. You can always find something in your favourite story which is not in other commonly-known stories and claim that this is evidence your story is exceptional.
The fact that some elements of the story are relatively constant is not persuasive evidence of a unitary historical founder either, any more than the fact that the basic story of King Arthur or Batman stays the same despite many reinterpretations is evidence that there must have been a real person behind those myths.
Perhaps you can explain where your reasoning differs from mine using the vaccination/autism example?
I don't have sufficient knowledge of medicine to have a discussion about vaccination and autism. Either way, it's also an irrelevant discussion.
Absence of evidence is not evidence of absence
Of course it is. But it has be weighed accordingly.
but absence of evidence after a search that should have found evidence if there was any evidence to be found is evidence of absence.
You've not given me any reason that it "should have found evidence"...
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).