C.S. Lewis, I think, failed to adequately account for the likelihood of stories propagating by exaggeration. Jesus need not have been a liar, a lunatic, or the lord, he could have been an honest, sane person to whom people ascribed claims of being divine after the fact (although as a religious leader setting up a splinter movement that strongly deviated from existing doctrine, I think the odds favor the historical Jesus having been at least somewhat crazy.)

I would say that the body of evidence posed by other religions suggests that, in the absence of a true religion, people will still make up stories of a religious nature (also, the degree of theological uniformity that exists among most existing strains of Christianity comes, not from the fact that early sects were at all unified, but that modern sects are almost all descended from the strain that killed the other ones off.) But my position is probably shaped to a significant extent by personal experiences with other people elaborating on outlandish lies that I came up with when I was young, with practically nothing to gain from it.

One of the interesting things about Christianity is that it's not using a probabilistic uncertainty framework at all. "Belief" in Christ is not just some confidence >30% in the godhood of Jesus- in Christianity, one simply believes or does not believe. This is part of why Lewis' logical dialectic has an appeal in that culture; it's basically Aristotelian, accepting propositions as True or False (this is also one of the reasons Thomas Aquinas is so revered for integrating the two in the first place, if I had to guess).

But, while this is the accepted inside-view way to approach the question of Jesus' divinity, it is probably a flawed way to interpret material experiences such as miracles. Note that even the Catholic church uses a formal system involving evidence and testimony, and thus in practice has a kind of rough 'confidence interval' for the truth of a miracle. Basically, you're stuck with the standard of actionable confidence.

Try to think of a precise answer to the question, just as an exercise. When miracles are the load-bearing component of your religious belief, it's going to come down to the degree of confidence that you need. If 40% is too high, what about 10%? 2%? For that matter, try to think about your current probability estimates in concrete terms as best you can, and notice when a given piece of information lowers that or raises it.

With Pascal's Wager on the table

One major problem with Pascal's Wager (among others) is that it doesn't specify which god. It applies equally to worshiping Yahweh, Kali, and Huitzilopochtli -- and offers no guidance on how to choose between them.

In this case the problem with it just being made up is that the witnesses seem too numerous and there seems to be too much extrinsic evidence, e.g. the records of his entrance into the hospital where the leg was amputated etc. However, it is still possible that it is simply an outlier -- a case of fraud even when fraud seems very unlikely.

Regarding your priors, I think this case is actually just like the other cases where you said you disagreed with Less Wrong; it is always a question of priors. The prior regarding people making things up is one of them. Similarly, I think your prior on the actual occurrence of extraordinary events is much higher than for the typical Less Wronger, and closer to the prior that ordinary human beings have.

So you could just assume that since Less Wrong is made up mostly of smart rational people who have thought carefully about their epistemology, it is more likely their prior is right, and so conclude that Christianity is false.

However, personally I think it is not that simple. When Eliezer said that he would prefer a machine that would destroy the world if God existed than one which would destroy the world at odds of a billion to one (I think it was a billion, not a trillion), I think that is extremely strong evidence that he is overconfident. So likewise I think it is clearly true that Less Wrong in general is overconfident that Christianity is false. Basically Less Wrong cannot avoid the standard pressures of a political community; just as Republicans are generally overconfident that it's ok to let people have guns, Less Wrongers are overconfident that God does not exist and that religion is false.

Generally speaking, in fact, Less Wrongers appear to have a prior regarding the occurrence of extraordinary events that is much like the prior scientists usually hold regarding such things. But that prior is in fact too low; this is why scientists took so long to admit the reality of meteorites and giant waves at sea, even after such things were sufficiently established by eyewitness testimony.

Of course, that does not mean that your prior is right; it just means the question is more difficult than simply accepting Less Wrong's priors.