My family is very Christian/Catholic. They're all super nice, very good people. I'm sure my cousin Ryan knows I'm gay (I post pictures of sexy women and go "WOO~!" and he's all like, "Yeah, me too!" xD), and I think he's cool with it, but I'm not sure how the rest of my family would feel if they knew. Hah, if they knew I was atheist, I'm sure they'd flip more. Ba, ba, black sheep, have you any wool? Advice~?
I think you should just ask them how they feel about it. Tell those who are excepting that, and those you feel won't except you- screw them. They should love you for who you are. Hope it helps.
Navigating families where religion plays a big part can be tough. I would deal with being a Lesbian before dealing with the religion bit first. Your family might be different and it might end up being a one two punch. Mom/dad, I am a lesbian and oh btw, that whole god thing, yeah, that isn't me either.. It'll be quite a bit for them to process. So break it gently...