What made you become an atheist?

What I mean was, was there a major catalyst that flipped the switch for you? I was raised a Christian. Becoming an atheist was, for the most part, a gradual process involving a lot of factors. It is pretty corny, but the final straw was when I first heard the song "God" by John Lennon. When I listened to that song for the first time, I wondered why I was still clinging to the tattered shreds of what little "faith" I had left. Has anyone had similar experiences?