I had an atheist phase when I was like 15. Probably lasted until my early 20s. I blame that on Carl Sagen, Richard Dawkins, et al. I obsessively read science books, and all of these smart people were telling me there isn't a God. They made sense; I believed them.

I think around age 20-21, I read Descartes' Meditations on Philosophy, which forced me to evaluate all of my beliefs about existence. Digging further into other philosophers (and religions), I realized there's not a whole lot I really know... about anything.

Eventually, I came to the belief of there being a creator, although who or what this creator is, I have no idea.