I can't speak to what convinced other atheists: it's far to diverse a group for me to say anything meaningful about their experiences. I can though talk about what convinced me to become an atheist.

For a long time I considered myself a religious person. I went to church every sunday, prayed regularly and believed devoutly in God. As I got older, however, I began to question the morality that was being taught to me. In Sunday school, my teacher posed the question: "Why do bad things happen to good people?" This question seemed obvious, so I responded that bad things happen to good people because bad things just happen, and that if the reward for good behavior was good things happening to you, then what was the point of heaven? I was satisfied with this answer at first, but when I thought more about it, I began to dislike this whole line of thinking. Do we only do good things because they will get us to heaven? That concerned me, because it made me think that everyone who did something good was only doing it for selfish reasons. I didn't like this cynical approach to morality that relied on rewards for good behavior and punishment for bad (a.k.a. Hell). Why wasn't being good "for goodness sake" good enough? This was one problem I had with my religion.

Like many other young Christians, I had been taught in Church that God created man and women in the garden of Eden after creating the world in 6 days. I accepted this at first, but it became harder and harder for me to defend in my mind the more that I learned about astronomy, biology and astronomy. My previous world view began to make less and less sense the more I learned. For about a year I lived with this strain existing between what I wanted to believe (religion) and what I knew to be factual. Eventually I had to take a long hard look at why I believed what I did. Really the only answer that came to mind was that it was the way I was brought up. This wasn't enough for me anymore. Just because ones parents believed something wasn't any reason for it to necessarily be true.

After facing that reality, it was only a matter of time before my beliefs associated with my religion were scrutinized and for the most part thrown out.

Now I don't want people to take this story the wrong way: there are plenty of people who can reconcile their scientific understanding of the world with their religious views, and I have no issue with these people. To me personally, that didn't make sense, and therefore I chose atheism instead. I still went to Church every Sunday (only recently have I stopped), and very few people knew (and few still know) that I am an atheist.

I guess the main reason I became an atheist was that I saw no good reasons to remain a Christian.

I think it is a transformative process, when you start to see irregularities in the bible that even apologetics won't soothe your mind from.

For example, you read that god condemns a group of people, and even though apologetics gives you drivel that god is above morality, that he is mysterious, that they are evil, etc. You cannot rest with this because your mind requires a legitimate moral reason which your experiences have taught you none exist. So you start to question the bible's authority because you wonder how such a god could be any good.

That's when you start to become atheist, when you lose the bias which forces you to affirm the bible. Then you can start to grow up.

Reading the bible.

If they still remain Christians after reading the damn thing you'd be wasting your time trying to instill some logic and reason into them. =/

As hard as this may be to believe, and as long ago as it was, I was once a christian. For me it was, that the stories in the bible looked eerily similar to may of the fables I heard as a child. I had never seen anything supernatural, so why is it that I should believe these stories and not the other ones? There are many religions and people belonging to other religions are just as certain of their religions, and since those people can be wrong what made me think Christians weren't just as wrong, especially when no concrete proof could be produced for either?

I think if a Christian has a child who has cancer and the Christian parent prays but their child dies, the Christian could then become an atheist.