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.
The feelings you get about this number are feelings that most any belief system would produce - you'd say the same thing with the same 5% if we replaced Christianity with Catholicism, for example.
So I take all beliefs systems as the reference class for that feeling. Smear that 5% over all belief systems - not just the ones that exist, but the possible ones that are as coherent and likely to arise as Christianity - and you should stop worrying somewhat.
As others have pointed out, probably 5% of the time you are mistaken, but that says nothing about the chance they are mistaken; unless you and them are mutually exclusive truth-tracking belief systems with all the evidence. Better assume you're perfectly spherical and updating takes place in a vacuum...
My numbers? The measure of worlds where a belief system like Christianity meaningfully alters my behaviour (how I choose to act in lots of situations, not just situations involving Christians) is something like one in ten thousand. As you make more specific claims (e.g. Bible says I'm going to Hell, I should act in accordance with {subset of Bible} ) I penalise by one in ten thousand, times one in the complexity of the statement you made.
To provide a more concrete mechanism for what you suggest:
5% is clearly a non-normalized probability. You can add up all those 5%'s, as well as whatever probability you'd apply to atheism, and then divide 5% by that sum. You now have a normalized probability that fits into a proper distribution.