My mother is a catholic, but not a serious one, she only goes to mass at Christmas and Easter.My farther isnt religious. However my mother made sure me and my brothers did communium and confirmation. However, she is tolerant of my athiest beliefs.
When i was about 8 and doing my first communium (sp?) it made me think about Christianity, and i decided that there was no evidence to say God existed in the first place. The only evidence was from the Bible, but i thought it was a bit odd that God was prancing all over planet Earth- from making manna rain down from the sky, burning bushes, the virgin birth and all and nowadays, nothing happens. Further more, the various miracles all seemed a bit far fetched to me- and the only thing to back them up is the Bible. So i decided God probably did not exist. Thats what i decided when i was around eight.
Now, ten years later my reasons are more sophicated, but that is how and why i became an atheist.