Well, and fantasy, too.
I recently read a very good article about religious and spiritual belief and how that fits into science-fiction and fantasy fiction. The article was really an interview, in slightly modified form, with award-winning author and well-known Mormon, Orson Scott Card. (You can read the article here: SciFiEditor.com.) The article brings up some very good points about why religion is often left out of modern fantasy and science-fiction, namely that the SF and Fantasy “Establishment” are practitioners of American Intellectualism, which “rebels” against religious thought and belief. I think he’s quite right in that regard. Also, I agree that such a view is quite unrealistic, considering how widely spread religious beliefs actually are in the world and how deeply they effect our culture and planet. Look at both the Crusades and the current political unrest in the Middle East. Indeed, in many places in the world, political strife is a direct result of conflicting religious beliefs and methods. As important as these factors are in our real world, can we afford to ignore them in a created world? I don’t think so.
In any case, I highly reccomend reading the article if you plan on writing speculative fiction of any kind. Religion in such settings is a very important, but overlooked, topic.