Christianity isn't a single proposition though. There is a lot of content there. A lot of unverified content.
The probability that you should believe Christianity because of some fear of hell (act on your worry) is something less than the probability that there is a God times the probability that God had a son given that there is a God times the probability that Jesus was the one and only son of God given that God had a son times the probability that there are souls given all of the above times the probability that God sends these souls somewhere GAOTA times the probability that God sends people who don't believe in him to Hell GAOTA.
Some of those things are pretty likely given the prior statement, but several of them are non-obvious.
And if you are going to act on a belief in a general diety... well, what policy do you take? He may only punish believers. He may only punish people who wear shoes. He may only grant souls to pillows and only send red pillows to heaven, blue pillows to hell, and all others (including mixes) to purgatory. Diety-space is HUGE and complex propositions without strong evidence are almost always wrong.
I am not even sure "Lots of humans believe it" counts as evidence. Is it more likely that something is true, given that lots of people believe it? I think the inverse is true: if something is true it is more likely that lots of people will believe it. But P(B|A)!=P(A|B)
I know you were speaking loosely, but you can't just multiply probabilities when they're not made independent.
A friend recently asked how strongly I believe that my deconversion from Christianity was not a mistake. Here's my response, and for those of you who are not Christians, I'm just wondering what numbers you would give:
"There is a part of me that wants to say the chance is far less than 1 percent. But when I consider what 1% must mean about my ability to follow complex arguments and base my judgement on the right premises, it seems absurd to say that.
Trying to honestly estimate the chance that I'm wrong about the Bible being generally reliable is a fascinating exercise... I know the number is low, but I'm not sure how low.
Today I would give myself a 1 in 20 chance of being wrong. If I were to consider the arguments of 20 other groups similar to Christian theologians, I would probably misunderstand them at least 1 time in 20. After talking with 20 groups that have a very different worldview, I might think they are all are mistaken, but once in a while, maybe 5% of the time, it would actually be me.
Wow, 5%!?! If I convert that into "There is a 5% probability that the God of the Bible exists and will send me to hell", I feel scared. But I know how to cheer myself up: I just say, "No way, the chance I'll end up in hell MUST be less than 5%. After all, the God of the Bible is CLEARLY just a big, mean alpha-monkey and... [rehearse all the atheistic arguments here]".
This back-and-forth from certainty to uncertainty makes me feel like I'm doing something seriously wrong.
So what about you? What chance do you place on some variant of Christianity turning up to be true, and what chance do you think a god of some sort exists?"
Numbers please.