You know there's no impossibility -- that's why I said "I assign less than 5% probability", I didn't say "I assign 0% probability". I'm talking about the probabilities of each scenario. Conditional and prior.
In that case substitute "highly improbable" for "impossible" in the grandparent and exactly the same argument still goes through. Why is it improbable for a cult leader to make up a story, and then later decide that they need to embroider the story further to reconcile it with some existing myths? It seems to me your argument-form also fits the following argument: "If Jesus was fictional then the Bible would say he was born at Christmas, but since it doesn't say Jesus was born at Christmas this is evidence he was real".
I just don't see why it's in any way improbable that one bunch of people made up the Jesus myth, then another bunch of people made an editorial decision of convenience that Jesus was "the reason for the season". In the same way I see absolutely nothing improbable about one person making up Jesus the Nazarene preacher, and then the same person or someone else later saying "Oh and by the way, he also fulfilled all these different prophecies in convoluted ways".
If you considered that, and you didn't treat "completely fictional" as the default, needing extraordinary evidence to decide against it, but having hardly any evidence in its favour, I think you'd reach the same conclusion as I.
The idea that "completely fictional" is the default position is a straw man argument. I lean towards the completely fictional interpretation because of the total lack of supporting documentary evidence, which I would find very surprising if he had existed given the enormous effort that has been dedicated to finding such evidence.
As others have pointed out, Jesus wasn't even an uncommon name and itinerant preachers weren't even an uncommon phenomenon. Yet we don't even have a decent bit of documentary evidence that could be a false positive. The prior probability that Jesus had some kind of factual basis is not incredibly low, but it's very low after we've conducted a major search for such evidence and come up empty.
Similarly the reason we are very sure that vaccines don't cause autism is that the hypothesis has been studied exhaustively and absolutely no sign of causation has been found. It's not that "vaccines are harmless" is the default position, it's that evidence for the non-default position has been searched for at great length and no such evidence has been found.
As others have pointed out, Jesus wasn't even an uncommon name
For real or for fictional people?
and itinerant preachers weren't even an uncommon phenomenon.
Doesn't that mean you should increase the estimation of the prior you have for him being real? You seem to be using it in the opposite direction.
What's the average amount of documentary evidence that an average real such itinerant preacher leave behind, so that we compare it with the amount of documentary evidence that Jesus left behind?
Unless Jesus left behind less "documentary evidence&quo...
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).