“World-building” is a challenge that faces many people: novelists, RPG hosts, screenplay writers, and most other categories of storyteller. Settings are the literal and figurative background of tales large and small, and for all that they are rarely the focus of a narrative the way characters and plot rightfully are, they are critical to that narrative making sense. Worldbuilding can be a forbiddingly large task, but it can be both efficient and rewarding if one keeps a few pointers in mind.
The classic Western-fantasy adventuring party, appearing across a wide variety of media and baked into what Dungeons and Dragons in particular expects adventuring groups to be capable of, consists of four very different characters. By default, there is a “fighter,” who wears armor and specializes in swordplay or another close-range martial art; a “wizard,” who is a combination mobile artillery piece, library, and miscellaneous magical toolkit; a “rogue” or “thief,” whose specialty is stealth, lock-picking, smooth-talking, acrobatics, and similar skills; and a “cleric” or “priest,” who provides the favor of the gods to their allies, usually in the form of magical healing and defensive magic.
That last person raises difficult questions about the overall shape of the fantasy universe, which every D&D setting tries to answer one way or another. It’s not difficult to imagine a fantasy world where the term “cleric” means something more like what it means in our world, and refers to someone an adventuring party might visit afterward for wound-tending and soul-cleansing rather than a steadfast and magical battlefield ally. Ivanhoe is probably the work of fiction most famously within this tradition. But most Western fantasy assigns clerics and other agents of the divine power well in excess of the demonstrated abilities of real-world religious figures, including the power to raise the dead on demand, instantly heal deadly injuries multiple times a day, and brandish holy symbols to disperse zombies. The deities of a fantasy world that is home to this kind of priest are, thereby, much more powerful than the god of Ivanhoe and any deity associated with real-world religious practice, and have far more direct and overt effects on the world at large.