One of the core concepts of any reality is the juxtaposition of the mortal and immortal, the living and the dead, and the base and the divine. Societies within that reality conceive of themselves in relation to others -- the world, other individuals and societies around them, and what may or may not exist beyond the world around them. Are they alone or unique in the universe? Have they made themselves out of nothing? Is there a transcendent referent toward which they may aspire, or is this all there is? In short, a world must have some idea about whether and in what form religion and faith will exist. The standard D&D model is a sort of bland, tolerant pantheism, where you find gods for almost every taste and specialty, and where everyone gets along tolerably well as long as alignments are not too opposed. Even if alignments are opposed, everyone stays out of the other side's way or doesn't cause trouble. In creating or implementing a world, however, a DM should decide whether that default assumption will hold true in his or her world. If you as a DM decide to try something different in this area, what follows might serve as a helpful guide in thinking about what that might look like.
It is possible, though difficult, to create a world where there is no faith and religion, where atheism rules the day, and where no one seriously accepts the idea of all-powerful (or even just super-powerful) transcendent beings. This creates a certain difficulty in the face of tangible manifestations of what is termed divine power -- priestly spells being the most obvious example. Such a world might also have a curious feel, since it would be very different from the world wherein most players reside. Whether players themselves endorse religious views is immaterial; the point is that they are aware that a great many people in their world do. To imagine a world where no one did might be jarring.
What would clerics and their ilk do in such a world? If they have no faith in deities, then where does their power come from? Perhaps native spirits that dwell in all things, living and dead, hold the secrets of divine power. Perhaps through communion with and understanding of these spirits, a divine caster can cause his or her will to be made manifest in a flare of divine power. Another answer involves the existence of a pool of divine power that equates to arcane magic in many ways, but that has certain unique characteristics that allow it to more efficiently do certain things (such as healing and protection). Those with the requisite skill and will could access it, though it might require a different mindset than arcane magery. Where arcane magic might involve attempting to figure out the intricate patterns of matter and energy that make up the world and then deftly manipulating the energy to create supernatural effects, perhaps divine magic might instead involve opening oneself wholly to what is there and imagining the change one wishes, then compelling it to be by sheer force of will empowered by the divine energies the wielder has channeled. Perhaps devotion and belief are required, but not invested in personified deities but rather within particular philosophies or mindsets or forces of the universe. The divine caster's absolute devotion and engagement with those forces or ideals could form the basis of his or her power.
Paladins are perhaps a useful illustration of this idea. Some could argue that paladins do not and need not follow any particular faith or creed in the sense of a religion per se. Instead, their supernatural and extraordinary abilities flow from their openness and total devotion to the ideals of law and goodness. Their willingness to fill themselves to the brim from the cup of lawful and good ideals and their absolute and unshakable confidence that what they are doing is right and true allow them to do things that others simply cannot. This isn't the way that everyone would run paladins as a feature of their campaign worlds, but it demonstrates the concept of how total devotion to a cause rather than an individual deity can become a plausible source for divine power.
WORDS TO THE WISE:
1. Do most people in the world believe in any deity? What is the religious landscape of your world? What portion of the general populace (or particular races) believes in transcendent or divine beings?
2. Are deities real? This question follows from the first: Belief is one thing, but the existence of something to back it is another. Note that just because the deities are real doesn't necessarily mean that mortals worship or even acknowledge them; they could be real and still have no one believe in them.
3. Does divine power exist? Even if deities exist and people believe in them, does this translate into a grant of divine power upon mortals? How frequently and directly does divine power make its presence felt? How many priests can actually use magic? Can they use divine magic, or are they arcane casters of some type? Do true clerics and druids (and other divine casters) exist? How common are they?
4. Do the deities get along? Assuming that more than one real deity exists, what is their relationship like? Do they even acknowledge the existence or divinity of the other? Are there families or alliances among deities?
5. What are their followers like? Are the followers of each deity friendly with those of others? Do all followers of an individual deity get along? Where are the centers of the faith? Are there regions where certain faiths are loved or shunned?
If we leave behind a world of total atheism, we can look at other possible cosmological ideas for how your game world works, and you can draw inspiration from the abundant body of fantasy literature. Robert E. Howard's Conan and Kull books include established religions in civilized and barbarian lands that are usually organized only in certain areas (and usually to some nefarious end). The priests who lead these cults, however, are often mere figureheads or charlatans, and manifestation of any real divine power is very rare and indirect. It tends to take the form of a sudden bit of good or bad luck that might or might not really be an act of the gods. Some of the "gods" encountered are simply monsters of some sort or creations of evil sorcerers. In such a world, the "priests" would be mostly experts rather than spellcasters. Any priests who can cast spells would be bards using their knowledge of ancient lore and minor magical talents to enrapture and convince the gullible sheep of their flock, or they could be adepts. A scant few would be sorcerers or wizards, plus perhaps the occasional druid or true cleric. The latter would probably be a lone prophet or a mystical hermit who the establishment considers dangerous or who is perhaps legendary for his or her healing powers. In any event, these latter folk would clearly be the exception rather than the rule.
Worlds like those found in Robert Jordan's Wheel of Time series or Stephen R. Donaldson's Thomas Covenant series do not have whole pantheons of deities, but rather an opposed dualistic pair. In both cases, the Creator has created the world and the pattern of reality, and the Dark One (Lord Foul in Donaldson's books) spends all of his time trying to break it. The manifestation of divine power is rare in the direct sense but fairly common in the indirect sense. The Dark One's machinations are more apparent because they are active; the Creator's efforts are implicit in the conservation of the existing reality and thus rarely show up in any tangible way other than as constraint upon or negation of the Dark One's efforts. Power-wielders fight on behalf of the Creator and the Dark One, but their power is mostly derived from a power source that is seemingly independent of either (though it was created by the Creator and corrupted by the Dark One).
Three basic belief options are available to residents of those worlds: the Creator is the good guy, and the Dark One and his minions need to be suppressed to avoid destroying the world; the Dark One is the good guy, and the Creator is a negligent slumlord who is keeping us all down, and things will be better when we can overthrow him and be free; the Dark One and the Creator are just vague abstractions out there and have no real bearing on our lives, so whether you choose to believe in either of them or not is mostly a matter of private and personal opinion. You can add flavors of each position (for example, the Children of the Light in Jordan's series, who believe that all magic is inherently evil). In general, however, neither world offers much in the way of organized religion; what belief does exist consists mostly of folkways, traditions, sayings, and superstitions, and each "side" is defined more by its opposition to the other than by its own theology. The conflict could be stated more precisely as the forces arrayed for and against the Dark One's side, since the ostensible agents of good rarely if ever claim to be working on behalf of the Creator; that is more of an incidental fact of their efforts to thwart the Creator's enemy. The relationship between the divinities and their agents is often tenuous and most frequently forms a broad boundary against what they should not do rather than what they should. The Wheel of Time d20 roleplaying game does not feature any divine-type magic wielders, and playing standard D&D in a setting with dualistic powers -- such as these, with a distant Creator and an imprisoned Destroyer -- could be difficult, though you could use any of the rationales available in an atheistic world.
Tolkien's world of Middle-Earth is generally of this same type, with a distant Creator in Eru and an imprisoned Destroyer, both in the original rebel Melkor/Morgoth imprisoned in the "outer darkness" at the end of the First Age, and his successor, Sauron, imprisoned as a shadow of his former self without the One Ring. We again see a world without any real organized religion and whose divine referents most commonly occur in songs, sayings, traditions, and other folkways. The Middle-Earth Role-Playing game did include divine-type characters, but really more as a concession to the way gamers have grown accustomed to playing (that is, with readily available divine healing) than because they really fit the game setting.
What differentiates Tolkien from Donaldson and Jordan is the existence of an entire absentee pantheon. To the residents of Middle-Earth, this pantheon takes the place of the absentee Creator as the presumptive "good" side of the divine equation, since the Creator himself is unknown to the general populace. Like the distant Creator of the other worlds, however, they virtually never insert themselves into the world (at least since the end of the First Age), instead providing indirect and implicit support to those fighting against the Destroyer. In a technical sense, the wizards and balrog(s) and creatures of that nature are minor godlings, but they have no real religious significance and thus, in a theological sense, are no different from Jordan's Forsaken or ta'veren or Donaldson's gravelingas or High Lords or white gold wielder. They are merely functionaries or agents for or against one side or the other. They are servants, but not proselytes for the cause.
David Eddings' Belgariad series (and its sequel) again has a distant Creator in UL and an imprisoned Destroyer in the sleeping Torak. Like Tolkien, the distant Creator is represented in everyday cosmology by a good-and-neutral pantheon of mostly distant deities. Unlike the other worlds, however, the gods of this world (besides being uniquely nationalistic in character) have full-fledged priesthoods, some of whom possess certain magical powers. Most magic-wielders do not seem to gain their powers directly from their divine patron, so it could be argued whether they were real "clerics" as D&D would think of them, but it's enough of a correlation that it could be used reasonably in a game with such a pantheon. As in the other worlds, most "priests" would be experts or perhaps adepts, but a larger proportion would be spellcasters of some stripe. Of more direct importance, the favored servants of the gods can call upon the aid of the gods, and the gods themselves can and do pop into the story from time to time to take direct action (or to directly empower one of their servants). Interestingly, the gods themselves (with the possible exception of UL) are also depicted as subservient to a certain higher cosmological law in the form of competing good and evil destinies for the universe, and this fits well with the standard D&D premise of a bounded planar framework within which deities must operate -- they are not above the law of the universe (with the exception of an "overpower," such as UL in this world or Ao in the Forgotten Realms setting).
Maybe more interesting from a DM's perspective is Eddings' second set of books, the Elenium and Tamuli, which contain a mix of divine traditions. The Elenes were monotheists (of a distant Creator), the Rendors fanatic monotheists of a somewhat different tradition, the Styrics pantheists, and the Zemochs yet a different kind of monotheists (worshipers of the destroyer Azash), plus the pantheon of the trolls. On the far side of the world were monotheistic followers of the Elene god but with a separate church hierarchy as well as the cosmopolitan empire of the Tamuls. There is a sort of ecumenism implicit in this cosmology, because all of the gods were real, and some would appear personally and talk about the other gods and their relationships or would interfere directly in the affairs of mortals when it suited them. The gods could teach the secrets of magic to their servants and would allow them to teach those secrets to people of other faiths. (The Elene church knights could and did learn the magical "secrets of Styricum," though some within the church objected to "polluting" the faith.) Hence, even though they did not often directly empower their followers, they could teach the followers the secrets needed to use magic. If this is the rationale by which clerics learn magic, then such a world would mesh perfectly with D&D; if the use of divine magic is more faith-based, then this model would obviously create problems.
The interrelationships of whatever deities may existare a key point in deciding how divinity will play out in your campaign world. In Greyhawk, Forgotten Realms, and Dragonlance, divine pluralism is the order of the day. Certain deities may be more or less prevalent in particular areas, but you might reasonably expect to find a follower of any given deity nearly anywhere. Geographic pantheons are rare (for example, the Suel pantheon in Greyhawk or the Mulhorandi deities in the Forgotten Realms), since the same deities are worshipped in locales thousands of miles apart. (However, in Dragonlance, the deities may have different names depending on the race worshipping the deities.) Racial pantheons are often universal for those races in a similar worldwide fashion, with the exception being Dragonlance. All followers of particular deities usually fit a fairly standard profile, and most believers are not persecuted for their faith unless they got into mischief, though they might not be fully trusted by those they meet who hold different views.
All of the above is the standard D&D model for how religions fit into the world, and it works perfectly well if the religion and cosmology of the world and its various cultures is not something about which it is particularly important to you to spend a lot of time detailing. If you are looking for a way to enrich the depth of your campaign world, however, you could spend some time pondering the way religion fits into the cultures of your game world. In creating a campaign world, you should think about how radically (if at all) you want to change the cosmological framework of your world and the deities (if any) that inhabit or influence it. Deciding what is happening in the heavens above (or the hells below) can be a great tool for framing the action that will transpire on the world where the adventure will take place, infusing it with meaning and significance. As a certain TV personality would say: "That's a good thing."
 The factions of the Planescape setting could be useful examples of devotion to a philosophy or creed that gird an atheistic divine caster's power.
About the Author
Jason Nelson lives in Seattle and is a full-time homemaker (taking care of his 7-year-old daughter, 4-year-old son, and new dog), a part-time graduate student (working on a Ph.D. in education policy), and an active and committed born-again Christian. He runs a weekly D&D game, having just completed a seven-year-long AD&D (heavily modified) campaign set in the Forgotten Realms. He began playing D&D in 1981, and he also plays in a biweekly campaign with a D&D version of his very first character (Tjaden Ludendorff, a psionic paladin) from 21 years ago!