I was raised as a Southern Baptist. My grandfather is a preacher - retired now. I used to believe quite strongly. My faith began to fade in my late teens.
One of my problems with religion is that there are so many of them. Practically every culture for all time has had their own separate religion, and each one believed that their's was "the one." What made me think mine was the right one and billions of other people throughout history were wrong and are burning in Hell? I just happened to get lucky enough to be born into a family that believed the right religion? That's bullshit. Clearly, religion is just an attempt to explain a frightening and complex world we don't understand.
On a personal level, I had a bad experience. I had been an active member of my church's youth group for years. I got my girlfriend pregnant in high school. You'd have thought I murdered someone. Many of the people treated me like crap about it. Not everyone, of course, but enough that it hurt. Where was the undying love and compassion they preached about? I sure could have used it at that time in my life.