First, when it comes to fantasy, religion is going to exist. Many writers create actual gods which walk among men and say “This is right, this is wrong, I do exist, and these are the curses and blessings you may expect.” If there isn’t that, there will be ancestral worship, animism, or a number of other simple religions at the least. You can even make it complex with a number of rituals, gods, and other facets.

Religion should be reflected by something which exists. These provide the best foundation, and from there it’s pretty easy to manipulate to the needs of the story. There are some obvious differences, though. Sometimes in fantasy the gods can actually exist. It’s possible in science fiction to reveal those gods as not real. Perhaps they were even killed. There are a lot of variations you can use. Just make sure it matters to your…