I went to a Jewish preschool, but I was never really indoctrinated, rather, I was exposed more to the well-meaning morals of that religion than a literal interpretation of its stories. At one point, I think around the age of 11 or 12, I actually questioned the existence of God. I'd never really thought either for or against it before.
I reasoned that if there was an all-powerful god, it could easily show itself to me if I asked it to. Soon after, I simply said to an empty room "God, if you exist, and you're really all-powerful, appear personally to me and prove your existence." After giving him a minute or two to do so, in which nothing happened, I concluded that religion was silly.
There is a whole group about this.