I've come to think that believing in God has something in common with smoking.  If you do seriously it before a certain age, it's very difficult, almost impossible, to quit later on.  It's not a willpower thing.  It's because it was ingrained in you before you could effectively question it.

When I look back at my childhood, I don't remember ever having a real sense of God's existence.  The idea never fully implanted.  I have had superstitious feelings, and numinous feelings, but never any real consciousness of a higher being.  I think my atheism was "there" all along.