Religion is one of the most important aspects of worldbuilding. It is found to some degree in every human society and is considered by many scientists to have emerged around the same time as language. Religion has had major effects on almost every aspect of culture and vice versa. It is innately interwoven with culture, and is vital to design if one wishes to make understood a constructed people’s worldview.
The first thing to do in developing a realistic religion is to consider the cultural backgrounds of the groups who share this religion, particularly the one in which it emerged. It is important to know first the geography and climate of the region in which it developed, as it profoundly affects the lifestyles and thus the beliefs of the people. It is clear that a religion developed in a hot, volcanic place like Indonesia would differ extremely from one born in inland Siberia. These people have vastly different worldviews, and their religions should be relevant to them. One must also consider how the people live within the society; are they farmers or hunter-gatherers? Are there cities or do they live in small communities? Is there a centralized system of government? If there is, how does the government function and how much influence does it have? If they have no over government, how are things accomplished? Is there a writing system? What are the artistic achievements? Are the elderly respected? Is youth idolized? How are the different sexes treated? What are the consequences of breaking out of the norm? How are conflicts resolved? These and many other things are important to consider.
Almost equally important are the relations with neighboring cultures. What have been the cultural influences from the cultures around the one in which the religion emerged? Have there been major recent conflicts or wars? What sort of goods do they import and export via trade with other cultures? Are there major minority ethnic groups within the society? What are the major religions in nearby societies? Is there widespread proselytism? Similarly, what have been the major historical cultures within the region? Do they consider a past society ideal and if so, what effects does this have on the culture?
Next, one must consider the reasons for the religion. Why was the religion born? Were the previous religions of the area no longer suited for a changing society? What were the catalysts for its emergence? Who have been the influential figures? What societal effects is it intended to have? Most religions have major unifying themes. What are the main tenets? Islam has its five pillars, Christianity and Judaism have the ten commandments, and Christianity has the golden rule. The Incas had “ama suwa, ama llulla, ama q’illa” as their major religious doctrine: do not steal, do not lie, do not be lazy. Not all religions have had such dogma, but in order for a religion to spread and adapt itself to different people, this is relatively necessary. What is considered important? How does one live a good life according to the religion?
This is a good time to think of the actual beliefs. This is perhaps the most transparent aspect of religion, and certainly the most obvious. What supernatural entities does the religion promote? Is it monotheistic, polytheistic or something else? What are the beliefs about the origins of the world? The sun? Any of the plethora of things important to the society? Whence came humanity? What are the mythical creatures, if any? It is important to realize that here you don’t need to make it more than simple animism, although you certainly may. This part of creating a religion is perhaps the most rewarding, but if one starts here it can be easy to create one that is not truly suited to its native culture. Think of the cultural things and doctrines that I talked about before and try to make your pantheons and myths consistent with them. If one’s constructed religion has a doctrine of peace, it is probably best not to have a blood-drinking war god. If its place of origin is in a harsh part of the world and there is a constant struggle for survival, perhaps a nature deity shouldn’t be a gentle and loving. Think about things that may want explanation and write myths to describe it. The society’s technological level is important here, as scientific knowledge tends to render mythological explanations obsolete. Also think of how nonbelievers are thought of in the society. If the society in which the religion develops has strained relationships with neighboring societies, it is extremely likely that those who don’t subscribe to the beliefs will be considered inferior, whereas in cultures with friendly relations it’s unlikely to be an issue if a person believes or not.
Next, think of the ceremonies involved. What are the holy days? These are most often tied to the seasons. How are they celebrated? Are feasts held? What are the rituals that are performed from need rather than celebration? Are sacrifices practiced? If so, what is sacrificed? Is dancing part of the ceremonies, and if so, what are the dances like and how do they vary? What about music? Ceremonial dress? What are the centers of worship like? How are people expected to act in sacred places or on sacred days? Here it is also important to recall the history of religious ceremonies in the region, and how ancient practices have changed —and not changed— in fitting into the new religion. If a religion holds nothing familiar, it is highly unlikely to catch on and thus will probably die fairly quickly.
Now consider the relationship of the religion with the community. Are people expected to be pious, or is the religion, overall, not very important? Are centers of worship also cultural centers? What goes on in the religious community that is not inherently part of the religion? Consider the people who are important in the community for their religious prowess, like priests and shamans. What are their relationships with the supernatural? How are they treated within the society? What influence do they hold? This is also a good time to come back to government; in many, many societies over the years, the leaders have been hailed as the descendants or incarnations of or speakers for the gods. If you choose something like this, you will also have to consider the level of respect given to them. Also, consider the religion’s stance on evangelism. Do they actively convert people? Here it is crucial to think back to the religion’s opinion on nonbelievers. Also consider how proselytizing religions fit into societies that they did not develop in. What effects has a foreign religion had on them? How has the religion had to adapt to be relevant to them?
Finally, it is vital to remember that religions change over time; humans misconstrue religious doctrines for their own means. If it exists, people will change it. Generally speaking, the longer a religion has been around, the crazier it is. Do not treat religion as a static entity in your stories. It is extremely dynamic. Every single aspect I’ve discussed and so many more things are capable of changing.