So, first off, I would agree that you're doing something seriously wrong, and what you're doing wrong is you're privileging the hypothesis.
Consider P1: "The God of the Bible exists and will send me to Hell."
Now consider P2: "There is a God, but the Bible is systematically wrong about God's plan. Everything the Bible says God wants, God actually wants the opposite of, and if I follow the Bible God will send me to Hell."
Regardless of how small or large a value I assign to p(P1), what really matters for my decision-making is p(P1)/p(p2). If that ratio is close to 1, then I ought not treat the Bible as my guide to how to act, regardless of either value. P1 could have a 50% chance of being true, and it still wouldn't matter.
That having been said, OK.
First off, like Emile, I have trouble unambiguously interpreting "Christianity is true."
Call C1 the set of all statements asserted by any Christian-identified theologian.
My confidence that the conjunction of C1 is false is roughly equal to my confidence that (a) I can in fact recognize logical contradictions and (b) two propositions that contradict one another are not both true. I don't know how to attach a number to these sorts of claims. I literally cannot imagine a scenario that would cause me to update my belief in that conjunction by a noticable amount. If the stars in the sky arranged themselves in a pattern that spelled out "C1 is all true, Dave!" I would still consider it vanishingly unlikely that C1 was all true. If I woke up tomorrow morning convinced that C1 was all true, I would still consider it vanishingly unlikely that C1 was all true. If I have to put a number on this, the number I put on it is zero. (Yes, I've read that article, and I'm saying this advisedly.)
At the other extreme, my confidence that the disjunction of C1 is true is, in the same way, one.
Call C2 the set of assertions about the actions of God described in the Old Testament as it has come down to us, starting with "In the beginning God created the heaven and the earth" and going on from there, including all the "and the Lord said to Moses" verses, etc. My confidence that there exists an entity of whom the conjunction of C2 is true is not quite as brainbreaking as the C1 equivalent; I can imagine events that would cause me to update that confidence by some noticeable degree. I still don't know how to calibrate it, though.
Call C3 the set of assertions about the actions of Jesus described in the New Testament. My confidence that there existed an entity of whom the conjunction of C3 is true is a lot higher than the other two conjunctions... but I still don't know how to calibrate it. Basically, we're still in the realm of "too small for me to conceive of".
My problems with "a god of some sort exists" are of a different sort... I can't quite figure out what that proposition even means. How is it different from "something exists", for example? (My confidence in that statement is very high.)
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.