That's false.
Let's say that we have 3 sets of propositions, A, B, and C. A is 40% likely. B is 90% likely if A is the case and 1% likely if A is not the case. C is 10% likely if B is not the case, 50% likely if A and B are both true, and 5% likely if B is true but A isn't.
B and C are clearly dependent variables. Nevertheless, simple math tells us that C is 8.17% likely, all other things being equal. Since Christianity is only true if all of the relevent propositions are true (as opposed to the scenario above) you can just multiply the probabilities together. (Unlike our A, B, C scenario, which also required some amount of addition.) You just multiply the probability within the conditional: that if God has a son that his one and only son is Jesus, rather than simply the probability that Jesus existed or that Jesus was the son of God. If Christianity could be true without Jesus being the one son of God, then addition would be required along with the multiplication, but this is not the case as the belief "Christianity" implies a zero-probability for these scenarios.
I somehow didn't notice the "given all of the above" shorthand.
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.