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.