I don’t know many people and haven’t talked to a great variety of them at much length. As such, I think my frame of reference for how people are, in a way deeper than observing them, is very limited. I know things people say and do, but not the way they think like I would my friends.
When I’m creating a character, which I’ve done some 1,000 times, I’m creating their psyche, not just their outward personality, and for me to do that, I’m limited to portraying a psyche which I can actually understand, unless the character is someone my character observes with their own slant and doesn’t really understand.
I don’t take that into consideration when I’m creating a character, but it’s my reasoning behind how it is that I’ve never written any characters who practiced any religion, real or fictional.
I know jack shit about what it’s like to be religious. I don’t know any religious people. I’ve obviously met a bunch over the course of my life, at times (in my youth) having put myself at ends with them, and later just avoiding the topic altogether in conversation. But I’ve never been close friends with one.
Does it matter that my characters aren’t religious? Not really. It’s always up to me what things are important to my characters and how much of that I even expose to the audience. It’s not like people expect to always learn the religion of a character anyways. Just I find it interesting that religion is something so totally alien to me that it never even occurred to me to give a character one.