While ordinarily, I might upvote something that is asking for help calibrating a belief, I think this has far too much potential to turn into an "I'm more atheist than you" fest. Especially when the title is "How confident is your atheism?" After reading a couple of the comments, I had that urge until I realized what was happening.
So I will be explicitly ignoring your request for numbers. But if you're really dead set on coming up with a number for yourself (I don't think the discussion is productive enough beyond estimation to, say, three or four orders of magnitude accuracy, but your call) I would suggest trying to think of other beliefs that you find as plausible as Christianity, ie., all of the other religions. You've got to necessarily have room for them in your estimation. Also, you should take into account reasons that creatures evolving in the ancestral environment would become religious.
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.