Depends on what you mean by "Christianity being true". If you mean "The miracles described in the Bible actually happened in the real world, and there is a supernatural God that cares about our actions and occasionally interferes in the world", then the chances are vanishingly small, less than one in a million.
On the other hand, if you mean "Following religious practice and giving priests a respectable position in society is good for individual well-being, as well as maintaining a harmonious and prosperous society; religious teachings are moral fables that help foster group coordination", then yeah, I'd put a much higher probability to that, though the exact value would depend of the religion being considered, etc.
I probably should have clarified to say, "the chance that Jesus of Nazareth is a resurrected God." I think all modern Christianities have this belief in common, and my estimations are based on this lowest common denominator.
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.