Introduction to the series found here.
Okay, quick disclaimer: When I say “religion,” I mean all varieties of faith and belief including atheism and agnosticism. Like it or not (and trust me, between the Bible belt and the internet, I’ve met plenty of people on both sides of the argument) belief is a part of human existence. Even if it’s not the focus of your story, it can go a long way to defining your characters, their values, and the nature of the world they live in.
When establishing the religious beliefs of your world, consider the following questions:
What sort of religion is it? Monotheistic? Polytheistic? Undefined?
Is it unified, or are there many branches? How well do these branches get along? Why did they separate? What are the differences between them?
What or who is being worshiped? If it’s an abstract figure, such as an omnipotent deity, how is represented in the physical world?
What is the origin story of your religion’s deities, teachers, or founding mythology? What other stories are significant to the religion’s core?
What is the source of the religion’s teachings? What sort of holy books or rules exist, and how are they passed down? Are they readily available to all devotees, or are they limited?
What virtues and societal rules does this religion hold dear? How are they enforced? What is the worst sin a devotee could commit?
What is the punishment (physical and/or spiritual) for breaking the rules? What is the reward (physical and/or spiritual) for following them?
Give an example of the very best sort of person this religion could produce, the most virtuous of them all. Make sure they’re a genuinely good person – even if your argument is that religion is evil. It’s important to understand both aspects.
Give an example of the very worst sort of person this religion could produce, the one who uses their faith as a weapon. Make sure that they’re faithful, but genuinely despicable – even if your argument is that religion is good. It’s important to understand both aspects.
If there is no centralized or defined religion, what values are upheld instead? How are they enforced?
How did your society come to be without religion – did they reject it, or did such things never occur to them? If the latter is true, how would they react upon meeting a separate, religious culture?
Again, keep in mind that these are just questions to get you started. The most fun religious portrayals I’ve seen are those that play with peoples’ expectations.
Christianity is a monotheistic religion worshipping an omnipotent god as defined by the teachings of Jesus, identified as the Son of God. It began as a centralized church but has since branched into many different sects, each with their own interpretations of the Bible. In broad strokes, Christian faith is based on forgiveness, redemption, and devotion. The most virtuous Christians are humble, charitable people willing to go the extra mile to help someone in need. The most corrupt persecute those who don’t live up to their personal standards.
In the webcomic Order of the Stick – which plays with traditional role-playing game tropes and D&D settings – there is no doubt that the gods exist, because they interact with divine-class characters on a regular basis. The gods are divided into four pantheons, each worshiped by roughly a fourth of the world; though some cultures separate from the mainstream, such as elves, have created their own gods. Each pantheon has their own rules and religious texts but, for the most part, they’re all devoted to upholding the balance between Good, Evil, Lawful, and Chaotic.