Stacey was raised in the Church of England, but she and her parents were most definitely Chreasters - that is to say, church on Christmas and Easter and don't really worry about it most of the time. Stacey believes in the basic tenets of Christianity, but she doesn't concern herself with it too much. Religion really is more of a special occasion thing than an everyday thing.
Sylvie is (half-)French, and so like a large majority of French people she was baptised Catholic and has hardly entered a church since. Sylvie's personal morals disagree a lot with the Catholic church's beliefs (such as...not being a slag), but if anyone asks, she'll say she's Catholic even though the few times she's sat through Mass (usually in France) she's been bored out of her mind.
Marietta is more agnostic than anything. She wasn't raised in any religion, but she believes very strongly in an afterlife (well, there are ghosts, aren't there?) where her mother will finally be reunited with her father. The whys and hows and who's in charge of anything are a complete mystery to her, but she's pretty sure there's something.