I've toyed with a lot of different approaches over the years in different campaigns. Beyond just a vanilla "the gods are real, they grant spells, don't worry about it" model that I tend to default to if nobody cares enough to make it a campaign point, I've also had:
* The gods are real, but they're basically the Marvel Comics version of the gods. They don't sit around granting spells to clerics - they're too busy fighting each other and saving the world from cosmic threats to care about what humans are up to on a daily basis. Clerics basically know the cheat codes of the universe to tap into the Power Cosmic that fuels the gods' power and as a part of that clerics have to "ritually embody" the principles that the god believes in if they want to cast spells - if they ever start doing things that are counter to their chosen god's desires, then they get cut off from the access. (Or worse, the god whose power they are tapping into notices what they're doing and gets mad about it).
* The gods are real, but they're basically the mirror of the Demon Princes or the Archdevils - very powerful Celestial beings but no more or less powerful than the top of the heap among the demons and devils. In that campaign clerics/paladins were basically warlocks in fluff though not in mechanics.
* There are no gods, and clerical magic is just a different flavor of magic. This was an old B/X game that I ran back in the day that worked out pretty well with the limited spell lists and fewer number of caster classes in the game - I've never had a desire to go back to it for a post-3.x campaign though.
* There are no gods - there are alien beings of unspeakable horror who grant their worshipers spells in exchange for devotion, which somehow they derive sustenance from. One of them delights in pretending to be a whole pantheon of "good" gods and feasting off the devotion of the poor deluded fools who worship him in one of his many guises. (This was a secret campaign bit that my teenage self thought was awesome but was basically me ripping off Lovecraft. It never got the reveal that it was supposed to get because the game petered out before it became a plot point. Honestly, it's probably for the best - the rest of that game didn't go near Cosmic Horror at all, so the reveal would have come out of left field. My teenage self was kind of dumb in many, many respects...)
* The gods are real, but they're basically the Marvel Comics version of the gods. They don't sit around granting spells to clerics - they're too busy fighting each other and saving the world from cosmic threats to care about what humans are up to on a daily basis. Clerics basically know the cheat codes of the universe to tap into the Power Cosmic that fuels the gods' power and as a part of that clerics have to "ritually embody" the principles that the god believes in if they want to cast spells - if they ever start doing things that are counter to their chosen god's desires, then they get cut off from the access. (Or worse, the god whose power they are tapping into notices what they're doing and gets mad about it).
* The gods are real, but they're basically the mirror of the Demon Princes or the Archdevils - very powerful Celestial beings but no more or less powerful than the top of the heap among the demons and devils. In that campaign clerics/paladins were basically warlocks in fluff though not in mechanics.
* There are no gods, and clerical magic is just a different flavor of magic. This was an old B/X game that I ran back in the day that worked out pretty well with the limited spell lists and fewer number of caster classes in the game - I've never had a desire to go back to it for a post-3.x campaign though.
* There are no gods - there are alien beings of unspeakable horror who grant their worshipers spells in exchange for devotion, which somehow they derive sustenance from. One of them delights in pretending to be a whole pantheon of "good" gods and feasting off the devotion of the poor deluded fools who worship him in one of his many guises. (This was a secret campaign bit that my teenage self thought was awesome but was basically me ripping off Lovecraft. It never got the reveal that it was supposed to get because the game petered out before it became a plot point. Honestly, it's probably for the best - the rest of that game didn't go near Cosmic Horror at all, so the reveal would have come out of left field. My teenage self was kind of dumb in many, many respects...)
