I've dabbled with various faiths in my games over the years, but for the most part I find having religious elements gets distracting. This is a game, not real life. It's a simulation, and that means a lot of things don't have to be represented -- at least, that's how I feel. If I were to incorporate religion as an active element, it would take time away from other activities, and that's a trade-off I'm not willing to make simply for the sake of "realism". At present, I do have one cult that is going around prosetylizing -- it's the FTP, the Followers of the True Path -- and it's based on the vampires. I needed something to do with them, so I'm letting them convert sims to their cult. I'm already growing tired of that storyline though, so the cult is probably going to break up very soon.
