There wasn't one single thing or sudden realization, and I'm glad for that. Religion is a complicated enough mess that it took me over a decade of careful study to decide it was, without a doubt, absolute bullshit.

My father was a youth pastor, so all of my earliest memories involve church or reading the Bible with him. However, my father was unusual in that he encouraged my early interest in science (I'm now a physicist). As a young boy most of my time was spent reading either the Bible, a sci-fi novel, or a physics book. Over the years I developed unsettling doubts about the Bible. I could show any rational person (though I certainly didn't know that was the term for it when I was 8) that the claims of science were correct. It didn't require faith, it only required that they look.

So I asked my father and the other pastors how one might go about demonstrating that the Bible's claims were true. Not realizing how much I was already doubting, they were actually quite thrilled. I was given a pile of theology and history texts to read, and I ploughed through them. Funny enough, around this time I can remember the senior pastor patting me on the shoulder and telling me that I was going to be one of my generation's great theologians.

In the end, no matter how much mental gymnastics these theology books presented, there was never anything provable, or, more importantly, falsifiable. By this point I was around 12 or 13, and I'd taken an interest in psychology and neuroscience. Naturally, most good textbooks on psychology take care to note that the crucial inherent flaw of Freudian psychoanalysis was that it couldn't be falsified. If confronted with something which might potentially contradict their beliefs, the Freudian could simply perform an elaborate bit of mental gymnastics to justify away the issue. I realized that I'd seen this exact same behavior before, in my family and in virtually every Christian with whom I had interacted. Learning about cognitive biases and the principles of good epistemology provided a framework for the gnawing issues I'd had with the church. When I was fifteen I first admitted to someone that I was agnostic. Now, I am an atheist, or, more technically, an "agnostic atheist" (one of my dearest friends is a philosopher, and he insists I add the qualifier).

I don't think these sorts of decisions should ever be made lightly. After all, what more important question can there be than the matter of reality's fundamental nature?

tl:dr Pastor's kid becomes physicist, learns too much about psychology, hangs out with too many philosophers - they're a terrible influence, always making you think more critically about things ;) - and turns out an atheist.