I'll try to assign probabilities to 5 propositions:
(A) I'm deluded, insane and unaware of this
0.03% of the population (that's 3 people in every 10,000) have Delusional Disorder. If I did have that (or various other disorders) I would be as certain as I am now that I was sane, so that puts an upper limit on how confident anyone can rationally be about their own sanity.
(B) I'm sane, but some form of supernatural exists (whether that's ghosts, an afterlife, supernatural karma, magic spells, or a sentient Gaia)
Given how hard science has looked for such things, for decades, this is the Black Swan problem. Were evidence of souls to become detectable to science, it would be an immense paradigm shift, easily comparable to the overthrow of the Geocentric worldview, a 1 in 1,000 year occurrence in science. None the less, it has to be possible to put a number on the chances of science itself being systematically wrong on a conceptual level. Let's say we can, at most, reduce the chance of this down to 1 in 1,000,000 because we don't have a long enough baseline to estimate the ways in which such things can be incorrect.
(C) Some form of deistic religion, whether that's Pastafarianism, Christianity or Hinduism, is correct in that an intelligent individual supernatural creator of the universe does exist (and some humans have attained at least partial knowledge of this Truth)
Rather than estimate this directly, I'd like to say that the chances of it being correct IF (B) is correct, are about 50:50, since it is one of the most common supernatural beliefs.
(D) Some form of deistic Christianity is essentially correct (an intelligent individual supernatural creator of the universe exists and picked a Jewish carpenter on just one planet in just one galaxy as his supreme messenger of Love and Truth)
Either astronomy is wrong, the Drake Equation (predicting other sentient species) is wrong, OR we're talking a 1 in 10^22 coincidence that I happens to be a member of the chosen species (and chronologically rather near to the key event: Jesus' presence on Earth). We've already factors in the changes of science being drastically wrong in (B). The sentient species on other planets is harder because of the Fermi Paradox but, even given that, based purely upon how near we are to the event compared to how long the genus Homo has been around (remember, even Neanderthals laid flowers on graves), you have to add in at least a 1 in 100,000 improbability factor for going from step (C) to step (D).
(E) The specific type of Christianity believed by a majority of Catholics (eg there is a Hell, and you will burn it in for an eternity if not 'saved', Adam&Eve, Noah, etc are real events not allegories) is essentially correct in most details
Talking animals? Unicorns? Global flood genocide by a loving God? Kill men who sleep with men? Get real. Let's throw in a further 1 in 1,000,000,000 for this step, and that's generous.
(A) is by far the most significant factor. If I'm sane, then odds of (D) or (E) being correct are astronomically low. And while from my own perspective I can only narrow down the odds of my own insanity to 3 in 10,000 someone reading this post or meeting me in the street has the benefit of an external view on that. And it is important to note that, even if I am insane, that doesn't mean that (E) is necessarily correct, just that I can no longer rely upon the accuracy of my estimates as to how unlikely it is.
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.