In a recent interview, Susan Jacoby described how and why people become atheists:
[U]nless you’re raised atheist, people become atheists just as I did, by thinking about the same things Augustine thought about. Certainly one of the first things I thought about as a maturing child was “Why is there polio? Why are there diseases?” If there is a good God why are there these things? The answer of the religious person is “God has a plan we don’t understand.” That wasn’t enough for me.
There are people who don’t know anything about science. One of the reasons I recommend Richard Dawkins’s book, The God Delusion, is that basically he explains the relationship between science and atheism. But I don’t think people are really persuaded into atheism by books or by debates or anything like that. I think people become atheists because they think about the world around them. They start to search out books because they ask questions. In general, people don’t become atheists at a late age, in their 50s.
All of the atheists I know became atheists fairly early on. They became atheists in their adolescence or in their 20s because these are the ages at which you’re maturing, your brain is maturing, and you’re beginning to ask questions. If religion doesn’t do it for you, if, in fact, religion, as it does for me, contradicts any rational idea of how to live, then you become an atheist, or whatever you want to call it – an agnostic, a freethinker.
I’m currently working on a book on a history of religious conversion. One conversion narrative is always like Saul on the road to Damascus. A voice appears out of the sky, you fall off your horse, you hit yourself on the head, and when you wake up you know Jesus is the lord. That’s the classic sudden conversion narrative. It doesn’t happen that way with atheism. People don’t wake up one morning and say “Oh God! I’m an atheist.” You don’t fall off a horse and wake up and say “Oh! There’s no God. Ah. Now I know.” No. It’s more a slow questioning, if you were brought up religious, of whether those things make any sense.