The panoply of gods in a typical D&D setting has always bothered me on some level, mostly due to redundancy. If there's a god of war that the Humans worship, do we really need an Elf god of war, a Dwarf god of war, an Orc god of war? Why can't we just say that there is ONE god of war, venerated by all races, and if one of them is especially warlike then that god is considered their patron? I toyed with this for a while, but it never really developed because it quickly became so stereotypical that if we were talking about real cultures I'd be accused of racism.
So the general idea languished for a while, until Trollsmyth linked to a series of PVP strips about D&D, and Jade's dialog in panel 3 really grabbed me:
"It's fantasy. It's a new mythology." Indeed it is. Why then do we feel the need to shackle ourselves to the notion of multiple polytheistic pantheons? Why not have something that's a bit more accessible to our Christian friends?
This is how many of my ideas develop: take an existing conceit, add some contrarian viewpoints, and let simmer until fully heretical. Thus was born what I hope is a fully-developed cross-cultural D&D religion:
The Light, The Dark, and the Gray
The Church of The Light
It isn't a church in the typical sense of the word in that it doesn't worship a deity. Instead, its followers worship and revere the Positive Material Plane, although they don't call it that -- to them it is simply The Light. It is the fount of all healing and the source of all souls, and when parishioners die they simply rejoin the glory of the Light. When priests turn undead or heal the sick, they are directly channeling the Light.
The Light, as a supernatural force, has a very simple agenda: Promote life. Healing, fertility (of both crops and animals), bounty; these are what the Light promotes. All "normal" cultures revere the Light. Who would be fool enough to reject a force that wishes to heal everyone, regardless of race or alignment? Even those with evil alignments like to have their wounds mended.
However, all is not puppies and roses. The Light fosters openness; truly, one cannot do harm to one's neighbor if his actions are made plain to all. The Light wants to burn away the darkness, prevent deception, and establish truth as an immutable concept. These are all admirable goals, but as they say, the devil is in the details...
Or in this case, the implementation. Mortals, as is their wont, have screwed things up. If the Light seeks to burn away deception, then it is the purpose of the Church to seek out all deception and expose it for the world to see, and bring it to justice if necessary. This means that "privacy" is often considered a sin by more extreme versions of the Church. After all, if you are doing nothing wrong, you have nothing to hide, and so you will have no problems if the nice Paladin enters your home and takes a look around. The Light Sees All, and if you try to hide something, then clearly you are a servant of the Dark.
Extreme adherents of the Light -- what other cultures would call hermits or ascetics -- do not live apart from the community. Instead, they are what we would call "naked, raving madmen." They live their lives in complete openness, hiding nothing (yes, nothing, develop that in as much detail as your depraved minds desire) from their community.
Churches of the Light are grand things, full of stained glass windows near the roof that capture the sun's rays from all angles, and large clear windows at the bottom so that everyone can see inside. They provide healing for free, but will not resurrect unless under the direst of circumstances, for that is "pulling a worthy soul away from its communion with the Light." The Church makes a tidy profit selling such things as Everburning Torches, Sunrods, and glass (indeed, they pioneered the art and still possess the finest glaziers around.) They're also excellent scryers.
Priests of the Light receive their spells at dawn.
The Cult of the Dark
You would think that the Dark would be universally hated in this setting, but it occupies a necessary social niche: people like their privacy. If the Church had its way, everyone would live in glass houses, and the entire town would know what you're eating and who you're sleeping with.
The Dark says, "Do what you want. We don't care." And that is very, very liberating.
If you're a young lover who is meeting her beau for a midnight tryst away from prying eyes, you are serving the Dark. If you plot to overthrow an oppressive tyrant in secret, you further the Dark's agenda. If you simply don't have the wherewithal to be honest all the damn time (and deal with the consequences thereof) and instead put on a "game face" when you're out in the world, and revert to who you really are in the comfort and privacy of your own home...
... you're a filthy lying Cultist and you should be ashamed of yourself! For your own good, the Paladins of the Light will kick your door down, drag you out into the street, and make you confess everything. You will feel so much better afterwards. You will.
The Cult of the Dark maintains that while the Church espouses honesty, only the Dark truly embodies it. After all, honesty is who you are when no one is watching, and the Light wants to watch you all the time. It wants you to be something that you aren't, forcing you to live a lifestyle you detest. The Dark doesn't care what you do. The Dark encourages you to do what you want. In the darkness, no one can see you, and no one will judge you...
... and from there, it's a very easy step on the slippery slope from "basic civil liberties" to "deviant behavior" to what is obviously evil behavior. If no one sees you sin, there are no consequences, and therefore you're free to indulge every little twisted desire you might normally repress. People who give in to the corruption of the Dark frequently become necromancers, assassins and blackguards, and those who die with its taint upon their souls frequently become undead. The Dark, of course, is the Negative Material Plane, the source of all destructive and necromantic power and cradle of undeath.
Shrines to the Dark exist in caves, basements, even dense thickets and copses of trees. They aren't blood-stained abattoirs or dens of perversion (though they can be); they can just as easily be a secluded glen where lovers go to escape the prying eyes of their parents. Wherever dimness and privacy reign, so too does the Dark. The Cult isn't organized like the Church, but it manages to flourish in individual cells. Cultists make a brisk profit selling wards and anti-scrying measures, and are the best fences in town. They'll even resurrect you... sort of.
Priests of the Dark receive their spells at dusk.
The Cabal of the Gray
Because sometimes people get tired of all this dogma and just want to be left in peace.
The Gray isn't so much a force or a plane as it is a compromise between the Light and the Dark. Yes, healing is good, but we'd like to keep our family drama private, thank you very much. Is that too much to ask?
By paying obeisance to both forces, they have created a third; a middle path of moderation, a gestalt entity, a "compromise god" if you will. If Light is Yang and Dark is Yin, then Gray is the line where they meet. Gray is the source of all other forces which are neither Good nor Evil. If you're looking for a place to jam elementalism, illusion, psionics, or anything else, this is the place for it.
Believers of the Gray aren't organized in any sense of the word -- there are no "Gray services" for people to attend. In fact, most people wouldn't even consider themselves Gray, just worshippers of the Light with some common sense and no time for dogma. Chores aren't going to do themselves, and while everyone needs healthy crops and livestock, sometimes you have to cut ethical corners to ensure that healthiness because the Church can't be everywhere.
If the Light is Neutral Good, and the Dark is Neutral Evil, then the Gray is just plain Neutral. It's the default belief system of most NPCs, who (conveniently) also happen to be Neutral.
Priests of the Gray come in two flavors: Druids who find all the Light-Dark extremism ridiculous, and wandering philosopher-monks who preach moderation in all things. Needless to say, the Church finds them dangerous heretics, the Cult thinks they're weaklings, and the common people are happy to take whatever assistance is offered as long as the Cabalists don't lecture them too much.
Priests of the Gray choose at character generation when to receive their spells: dawn, noon, dusk, or midnight.
Hopefully I've fleshed these ideas out enough that they make sense and provide multiple uses within a a campaign. I've tried to balance them out, putting a little evil within the good and a little good within the evil, just to make things interesting. It's all in how they are used... the Dark can further good by protecting rebels who oppose an evil (if law-abiding) tyrant, and the Light could seek to expose their conspiracy, while the Gray goes about its business helping (or hindering) both sides as convenient.