Religions have many different aspects that should at least be given thought if not careful consideration. Use these to guide your creative process when developing new religions and deities.
Deity/Pantheon: Your religion does not need to necessarily have a deity, and it can even have an entire pantheon. I would venture that while a trained priest might perhaps specialize in one deity, a religion can have many.
Dogma: What are the principles and teachings of your religion? What does the deity implore of their worshippers? What is and isn’t allowed? What are the ethics of the religion? Why must we follow these principles?
Symbols: As important as the religion’s dogma are its symbols. How is your religion recognized on flags, tabards, armor, weapons, artwork, and holy symbols? Does your religion have a holy color or color scheme that they could use for their priestly robes?
Temples: Where are the religion’s places of worship? They could be secluded and secret or in/near cities. What do they look like? Are they merely household shrines or grand cathedrals? Do they have any distinguishing features?
Rites and Rituals: What sorts of special ceremonies do the clerics of your religion practice? Are there any special material components that have meaning for the religion, deity, and ceremony? How long do ceremonies take and what is supposed to come from them? Rituals always serve a purpose, even if that purpose is merely affirming your faith. Rituals are useful as story elements as well as for players to perform.
Affirming Faith: telling your god you’re there and in prayer. It can be as simple as a daily prayer or weekly ceremony or more in-depth like a monthly or yearly ritual.
Proving Devotion: proving your faith to your god, usually meant for those who might be in doubt or who have wavered.
Initiation: rituals for new members to the religion.
Induction: rituals for new clergy members or clergy moving up in hierarchy.
Satiation: your deity demands sacrifice of something valuable to you or to it.
Boon/Blessing: the ritual seeks something of your deity, perhaps a bountiful harvest or victory in battle.
Magic: a ritual might be held to cast certain spells or perhaps to increase the power or scope of a spell. These can also be used in creation of magic items.
Healing: rituals for performing healing magic.
Funerals: ceremonies for the dead.
Marriage: ceremonies for binding individuals together spiritually
Holy Days: Often rituals can coincide with special days or times of the year. Holy days can be predicted and often signify important seasonal or historic events. Harvest, springtime, solstice, and equinox holy days are common, as are those commemorating the deaths of martyrs or important dates in the religion’s history.
Myths/Legends: Are there any stories or parables that your religion teaches? What stories of the gods do they tell? Do they have any specific myths relating to things like the creation of the world, the creation of elements, the invention of everyday things, or perhaps the invention of morality?
Prayers/Sayings: To help you roleplay priests of this religion, you can come up with some common greetings, farewells, and blessings that might be associated with the religion. “Pelor shines upon you” and whatnot.
Titles/Hierarchy: What are the ranks of the clergy and do they have any special titles? Are there any notable NPCs in the religion’s hierarchy? For instance, those that worship Mammon, the archdevil of greed are often called Covetors.
Clergy: Do the clergy perform any services for the rest of the population? Usually this involves healing or holding ceremonies, but they could have a broader scope in a theocracy or a narrower scope if secluded or unpopular. What do the clergy look like and wear? Do they favor certain classes other than clerics?
Worshippers: What sort of people are drawn to the religion? Are there certain races, classes, or kingdoms that worship them? What convinces them to follow the religion’s dogma? Is it out of fear, necessity, protection, comfort, or prosperity?
Relations: Does this religion have allies or enemies? These could either be allied or rival churches, deities, or religions. Furthermore, certain kingdoms or people could ally with or oppose the religion. Think of how each faction and religion in your world sees this religion.
Art: How does your religion express itself? Define your religion’s art, architecture, fashion, and songs and how they compare to other religions and cultures.
Relics: What sorts of holy relics belong to your religion? These can be body parts or objects belonging to important martyrs or high priests or heroes that champion the religion. These might be kept in temples or may have been lost to time. Perhaps some are magic items being used by chosen (or unscrupulous) adventurers.
