Sorry, the site I was using kept changing and I haven't had time to sort it out yet.
Religion plays a role throughout the comic because it's the center of some of the characters' moral decisions. It might get referenced here and there, but it's not a central theme.
Apen's favorite saint is Peter, because he felt unworthy to be crucified the same way as Christ, and so he wanted to be crucified upside down. Apen really connects with that.