When I first read Ursula K. LeGuin's novel The Left Hand of Darkness, one of the things that really impressed me was how she'd given a religion to the people of her world. It was one of many things in that book that made her world feel real.
So today I thought I'd share some thoughts on using religion in your worldbuilding.
Question 1: Do you need it?
Answer? No, not necessarily. One of the reasons that religion can be extremely valuable in worldbuilding is because it provides a foundation for people's thoughts - a set of metaphors and references that the community can share. These metaphors and references can infuse the entire community, and give your world a sense of common culture.
However, religion in and of itself is not necessary, because a similar function can be served by what I'll call a "world concept." A religion definitely provides a world concept to its people - one in which a sense of the sacred is inherent. But world concepts don't necessarily need to include the concept of a deity or of sacredness. It depends on what you're doing with the story!
My two Analog stories are a case in point. In "Let the Word Take Me," religion was terribly important - indeed, the nature of the Gariniki beliefs was central to the resolution of the story problem. In "Cold Words," I deliberately chose to give the Aurrel a world concept that did not include a deity. Their "natural order" was basically all about the food chain, and it fit well because they were carnivorous creatures who had come out of a hunting culture. I wanted people to concentrate on the linguistic question at the center of the story - which fit well with their world concept - without getting distracted by questions of deities.
You might wonder what I mean by "distracted." Well, since this is sf/f, whenever you have a religion involved there comes a moment when the question can come up: "Is this mystical force real?" I know it came up in "Let the Word Take Me," but it would have been completely irrelevant to "Cold Words," which was why I left religion out.
Question 2: What are the consequences if you choose to include it?
As I've just said, one possible consequence is the raising of questions about whether the deity or mystical force is real. But it goes much further than that.
If you create a religion for a society in your world, then it has social consequences. The infusing of metaphor and world concept into your story can be greatly advantageous, but can also require a lot of work. Consider some of these questions: How does the religion influence people's concepts of life and death? How does it affect their sense of morality and consequences for their actions? How does it affect people's daily personal practices? How do different people relate to the taboos on behavior and language that are often associated with particular religions?
Furthermore, in choosing to have a religion in your world, you are committing yourself to a history of religion for that world. Is there only one religion? Why? How and when did it arise? If there is more than one religion, how did that come about? How do the two-or-more religions relate to one another and why? How does religion relate to government? Are the two congruent, or separate? Is there tension between them?
Finally, when you have a religion in your world it's important to consider the different ways that individuals relate to religion. There are people who are involved in the practicing of the religion, and laypeople. There are those of great faith and those who question aspects of their faith or the details of scripture etc. There are those who reject religion entirely, and they may do so for a wide variety of reasons. And don't forget the secular members of society - the people who will not engage in regular ritual practices associated with a religion, but who share its metaphors, follow the same moral precepts, think in terms of the same world concepts and may even swear by its deities.
When it comes right down to it, I'm not going to make a simple yes-or-no recommendation about including religion in your world. I do recommend, however, that you consider what its place might be in a society you're building, and very importantly, consider what its place would be in your story. You might build a religion into sections of your world, but if it has little relevance to the main conflict of your story, you should probably keep references to it very low-key. On the other hand, religion might be just the tool you need to guide the principles, judgments and actions of a key character - in which case, by all means go for it to the hilt.
Make the choice consciously, and your story will probably be stronger as a result.