Sure, I can say something. Even as a Christian I was, in terms of the actual content of my beliefs, an atheist for all intents and purposes. The only reason I didn’t call myself one* was because I identified as a Christian and, as an extension of my liberal Christian theology, had this idea that the theistic “language game” had a valuable function for at least some people.
Lately I’ve come to realize I have no idea what that valuable function is, and that even if it has some valuable function for some people, it just doesn’t have one for me except the function of “letting me identify with a particular social group that I feel sentimentally linked to” which, if I value that, I really shouldn’t.
Moreover, I had a good hard look at the question of what exactly we can know about Jesus with any reasonable certainty and realized that, as far as I can tell, we have basically no good reason to affirm any particular claim about what Jesus thought or taught. Well, if as far as I can tell we have no good reason to affirm any claim like that, then what sense could it make, really, for me to call myself a Christian?
I mean, I can try it. I can argue that the values I have as a result of what makes me Christian urge the kind of intellectual honesty that affirms, when the evidence seems this way to me, that I really have no idea what Jesus was about. But that’s as much as to say that my Christianity points away from Christianity, isn’t it? (And indeed, I’d affirm that claim.)
So. The transition came about because I came to think I have no reliable reason to think I know anything about Jesus, (hence no Christianity and really I was only a theist of sorts by dint of my identification with Christianity) and because I came to think the theistic language game has not demonstrated itself to have a valuable function. Indeed, I have come increasingly to think it is pernicious, giving people excuses for certain kinds of intellectual and rhetorical dishonesty that we really should be fighting against. That’s putting it a little too strongly though–even now I do think there are plenty of people for whom an honest theism exists and who are right to stand by it, in their own particular circumstances. I just don’t think I’m one of those people, and I think there is a better ideal we can point to.
*though I did sometimes, in some conversations, go ahead and say that my beliefs amounted to a kind of atheism