Ask OB: Leaving the Fold
I was in a similar situation, though not quite as harsh - I was a child in a deeply religious family. I managed to get out without loosing everything. The key was to do it gradually. Even though I realized religion was bogus when I was 15 or so, I didn't immediately come out and say "Guess what guys, I'm atheist now!". To do so would have been foolish. I just showed less and less interest in going to church, bibles studies, and so forth, and whenever my family was having a discussion, I'd take the "devil's advocate" more and more. Then, by the time I finally did come out and say "Guess what guys, I'm atheist!", nobody was all that surprised or outraged.