Just as background info ; my parents are Catholic, both Polish (originally from Poland but ended up, individually, in England after WWII, met & married there, had me, and moved to the USA when I was 9). I am an only child, but the survivor of a pair of premature twins. My mother also had a premature baby boy when I was about 4 who did not survive either.
My parents were fairly devout Catholics (who have seemed to become even more so over time) when I was growing up, but, like xyzzy, I somehow filed all the Bible stories, etc, in my head along with myths and fairy-tales, and was almost traumatized to find my father kneeling at the foot of his bed saying his prayers one evening, as it had never occurred to me that doing so was anything more than a childish bedtime ritual that people would outgrow along with lullabies and bedtime stories. And I was very young then (maybe about 6?). I think that was something which solidified a sense of non-belief in me, weirdly. I mean, one would assume that seeing one's parents' and other role models' faith would strengthen that in a child, and give support to the idea that it was the truth, but in my case, it just seemed to serve to puzzle me that all these people never seemed to question something which seemed so unbelievable.
For the record, as an only child, I was very close to my parents, but maybe to the point of feeling a bit smothered. Especially as I got a little older. But I also seemed not to have inherited their personalities to any great degree. They, especially my father, tend toward being very conservative, and while I was not a wild child or rule-breaker, that was probably in large part due to my being paralyzingly shy. Because I was a total Bohemian at heart.
Interestingly, my parents both had older siblings who had also ended up in England, all with English/Irish spouses, and remained there after we moved away. I only knew my cousins slightly, as we had not lived close by even before our move. It was not until visiting England with my daughter some 14 years ago that I reconnected with a few of those cousins in a bigger way, and found almost eerie similarities in our world views/philosophies/religious skepticism. My father's brothers had both died some time earlier, and though as far as I know they pretty much had continued going to church, the middle brother in particular was of much more an agnostic bent. So it does seem to me that while upbringing plays a part, there's a lot of genetics involved in a predisposition to believe or not. How else would I have ended up so much more similar, personality-wise, to uncles and cousins I barely knew than to my own parents?