I grew up Roman Catholic. Baptism,First Communion, Confirmation, the whole bit.
When I was very young I really believed in Santa Claus.
At some age, I went through a phase where I wanted to believe, but it was just too preposterous, and the evidence for it was flimsy. Finally, I realized that such a being did not exist.
A few years later, at about age fifteen, I applied that same process to the Christian God. I also read everything I could find by Robert A. Heinlein. I learned about Zeus, and Odin, and realized that the Jewish mythology was at that level of credibility.
Then, I read the King James Bible, cover to cover.
Atheist ever since.