Well let's see. I've been giving this some thought.
Carlisle: Well he was a form of Protestantism; Edward explains that he was an Anglican minister in the 1600's.
Edward: Some kind of Christianity; probably Catholicism seeing in how much he believes in heaven, hell and especially purgatory.
Jasper: Some form of Christianity; maybe a Baptist, Methodist or Presbyterian seeing as those were the predominant faith groups during the mid nineteenth century. Probably raised but not practiced.
Alice: I picture Alice as a Pagan. Someone who's more in touch with the Earth. A belief that many gods control the seasons and crops and weather and such. (That also could've have contributed to her being commited, especially during the time period she lived in *ponders*)
Rosalie: Yeah... right. Atheist or a Nihilist (that's more of a philosophy though) maybe. Although this may not have occurred until after her transformation. Being an immortal vampire, incapable of having children or a normal life doesn't really strike a fancy with her. That could have spurred her into not believing in a god.
Emmett: Emmettism... ROFL aka HAKUNA MATATA. Agnosticism. Just a belief in some god. Or
Determinism. Again more of a philosophy; belief that things just happen and not necessarily for any divine reason. Everything we do is determined by "an unbroken chain of prior occurrences"
Esme: I'm really having a hard time with her. Now, I think she's a little more independent than to just do whatever Carlisle does. But I can't think of a religion she would've followed in her mortal life. I could find historical evidence and make an educated guess, but those would all probably be some form of Christianity. Personally- I have no reasoning behind this, but I just always got the
feeling that she was Jewish. I don't have any proof; that's just my impression. (a lot of people seem to leave Esme out of there considerations
. Well, I say "yea for Esme"
)
In the end I think the only Cullen's that actually practiced a religion would be Carlisle (obviously, he was minister), Edward, and maybe Esme. Although Edward only really dwelled on his potential lack of soul, not that the God he believed in (at least the God I think he believed in) would forgive the sins of anyone who was truly repentant. I got an inkling throughout the series that Edward might've been a bit repentant, anyone else?