They may all profess that belief, but is it the real reason they're Christians and not atheists? What if metaphysical claims are just identity markers, that Christians hold because they are completely abstract and divorced from reality?
The "religion is good for social and individual well-being" argument is a bit of a steel man; it's easy to make fun of stupid arguments by Christians, but maybe Christianity is actually beneficial for complicated reasons, and many Christians see the benefits but mistakenly believe in simpler reasons (and those that can articulate complex reasons are ignored, because complex explanations are boring and look like tortuous rationalizations).
The important question shouldn't be "is such-and-such point of Christian doctrine factually true?", but rather, "is it better for me and society that I identify as an atheist rather than as a Christian?" . Reducing the second question to the first is a cheap way out. Sure, there are good arguments for why secularism is better than religion, but those are waaay less overwhelming than the arguments of physics and biology (and frickin' common sense) over theology.
Which is a special case of "What falsehoods should I prefer to believe to their corresponding truths?", which is a far broader question.
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.