Gods. Spirits. Demons. Supernatural entities, or maybe mere myths. Omnipotent creators, powerful beings who occasionally interact with lesser ones, or impersonal concepts. Saviors, benevolent helpers who offer illumination or practical assistance, aloof observers, petty mischief-makers, or evil presences. Whatever they are, humankind has always been obsessed with them and with their demands: compliance with their rules, offerings (up to human sacrifices), and more. Therefore, long-lasting organizations, epochal social and cultural phenomena, and even states have revolved around religion, i.e. the relationship between humans and their gods. Even non-human intelligent species in fiction often have at least ethics, or belief systems.
The more difficult question isn't: "How can you use religion in a role-playing context?", but: "How could you not use it?".
This supplement is designed to provide both game masters and players with basic tools, guidelines, and ideas for using religion in a role-playing context. It will give depth, flavor, and realism both to campaign design and character development.