Religion–or even lack of religion–is something that will deeply affect the way people live in your world, both individually and as collective cultures, so let’s get into the basics of building a religion.
Who Is God/Who Are the Gods?
Is the religion monotheistic or polytheistic? If there are multiple gods, what are their domains? If there’s only one, is he all-powerful? What are their personalities like? Are the gods really gods, or are they people (maybe superhumans) who have been raised to god status and worshiped? How do the people who follow this religion (or those who don’t) feel about this? Are they invisible or visible? Do they meet with their followers? Has this changed over the ages? Do they seek personal relationships with their followers or are they impersonal and distant?
How Do People Feel About Religion?
Do people who follow a certain religion accept people from other religions, or do they avoid them like the plague? Do they think the other religion is a blight on religion? Do they believe their religion will be the only religion someday? Or even that a different religion will someday be the only one? Do they really have faith in their god(s) or do they simply follow because of their family, or their culture, or because they have nothing better to believe in? Are there people who don’t subscribe to any religion? Why? What happens to people who convert from one religion to another?
How Is God Worshiped/How Are the Gods Worshiped?
Do they require sacrifices? Festivals and feasts? Are they prayed to? Sung to? Do they have religious texts to be studied? Do they expect people to meet with them personally?
How Does Religion Influence Everyday Life?
Religion influences people’s decisions, and culture’s decisions. Does the culture that serves a god of growing things have an abundance of gardens? People who seek to nurture everything they see? Does this carry over to children, leading to large families? Are non-gardeners looked down on? Do the followers of this god disdain those who follow the god of destruction and war, viewing them as uncultured fools? How would these followers’ views change if the god were personal vs. impersonal?
Fictional religions are a worldview issue for the author, as well as for the world. What you believe in will affect what sorts of religions you’re comfortable developing, and your worldview will end up woven into the final product. Keep that in mind as you develop your world’s religion(s).