@GoshisDead,
Just as a side comment, I was raised in a luke-warmly religious family (found out my mother was actually agnostic at around the same time I ditched religion) but got sent to a church-based school.
So...I was taught all the christian stuff, but for me doubt came way earlier than you are talking about.
It wasn't so much initially that I doubted the whole god thing,. but I was struck by how little this apparently astounding information affected everyone! It seemed as though, if all the Jesus stuff was true, that people should be walking around with very little in material possessions and be focusing on loving their neighbours, giving to the poor, and meditating on the world to come.
Then, as a kid, I loved myths and legends from all cultures, and also Mary Renault's books on ancient Greece.
I recall one day, when I was about 8, perhaps, being absolutely struck by the fact that other peoples believed in their gods and just as firmly as I was taught to believe in "mine".
That was the beginning of the end.
Reading history kind of did the rest.