Here's an excerpt from what will eventually be a character handout for my campaign which I'm currently designing:
The campaign lacks a coherent pantheon – some clerics are servants of a single deity, some of several at once, some of philosophies which may or may not incorporate the gods and goddesses of the world into their doctrine. While no religion is monotheistic in an exclusive sense, some religious orders claim primacy for their god(s) over all others. The clerics of the World Mother, for example, claim that She is the creator of all things, including other deities, and especially the natural world; there are, however, druidic philosophies which do not recognise any deity of the force(s) of Nature at all.
The gods apparently do not require doctrinal purity in return for the favour they grant their worshippers – most clerics agree that this is an accomodation born out of the gods’ conviction that mortals’ believing part of the truth is better than none, and may lead them to the whole in time, though extremists of all stripes suspect that those clerics who dissent from the truth are merely drawing their power from the elemental forces of the universe loosely associated with the god they claim to follow, as clerics of philosophies do.
Basically, I'm trying to add a little more real-world flavour of competing religions and philosophies while still maintaining the actual, provable existence of the deities that D&D assumes. I was a big fan of Planescape back in the day - my favourite and longest-lived character was actually a Third Edition Planescape PC - and so I suppose I'm influenced by the existence of factions such as the Athar and the Believers of the Source in that setting. I may even have small cells of those factions and others like them in my campaign, come to think of it, since I want to make the planes accessible.
The main change that I suppose I am making is that it will not be possible to discern the "truth" about the gods and their relationships through mortal means. I like the idea of clerics who stray from the "accepted" dogma being accused of no longer drawing their divine magic from their gods, but from the multiversal forces of Good and Evil, Law and Chaos, et cetera, but I also like the fact that the truth of the accusation is impossible to uncover.
The complexities that arise could be fascinating. Do clerics always receive visions of the gods that they worship when praying for spells or casting commune? If they do, but they are accused of straying from the "correct" dogmatic path, does that exonerate them, prove that the gods don't require a precise set of beliefs, or suggest that it's possible such visitations are not necessarily "real"?
Perhaps clerics of some religions see visions of their gods, and clerics of others do not. It would be entertaining if the clerics of the World Mother (for example) never saw a personification of their goddess, but some of the druidic sects which don't associate themselves with a deity of Nature do see personifications of Nature from time to time - not necessarily a god, but who knows?
Mmm, delicious mixed signals.