If you have ever ventured to build a homebrewed campaign world, you've certainly been pestered by those players with cleric and paladin PCs (and the occasional players of other classes who are heavy on role-playing) about the religions of your campaign world. Even if you're running a published campaign setting, some of the more "devoted" cleric players might be hungry for deeper details about the existing religions. If you've ever been in one of these situations, you probably wished for some guidance on how to flesh out your campaign's religions.
What I'm going to give you here might possibly not meet your expectations, since I'm not giving out guidelines for designing holidays, rituals, symbols and other little details. I'll be working at another level - giving you ideas on how to break the stereotypical molds of D&D religions, in order to make your homebrew's faiths sound livelier and more believable, and even spice up those of published worlds by changing them around a bit. This article details the different kinds of religions or metaphysical beliefs that are known to man, with real-world examples; the way they tend to view other religions (especially when they disagree on how the world works) and their followers; and how can they be used with D&D classes, especially the "religiously-inclined" ones (divine casters and monks).
When applying this article to your campaign, you might want to pick one single religion (or metaphysical belief) as the "only true one", and therefore let only the divine casters of that religion have spells; it would be painfully obvious that the other religions are false, and the world would most likely end up being monopolized by that one religion. Alternatively, you could make several different religions that have fundamentally different world-views, like in the real world - in that case, for example, the elves might be pantheistic (see below), while one human nation believes that there's a single deity, and another has several deities that they pray to. In this case, you could either select which one of them has the right worldview (or maybe decide they're all wrong), or even say that they're all right in their own way, and find some way to deal with the inconsistencies. Both the Planescape campaign setting for 2nd edition AD&D, and the Mage: the Ascension game for Storyteller, might give you good ideas for that...
As a last note, remember that these are just the author's opinions and personal classifications, not claiming any sort of scientific authority; besides, they are not meant to either value or belittle any of the different religious options, or judge their accuracy or worth in any manner, but just to explain how they work in a simple way that can be applied to D&D campaigns.
And now, on to the religions!
Independent Polytheism
This is the kind of religious belief that is usually present in most D&D campaign worlds. In this world-view, there are several deities that are independent from each other - some of them might be more powerful than others, and some deities may form associations or even be subordinated to another deity, but there's no all-encompassing order or society that all deities fit into, and no single deity is greater than all others. Different communities will be usually drawn to different deities, but all deities are generally recognized everywhere. The closest real-world example would be Greek-Roman polytheism; while the twelve Olympian deities were organized under the rule of Zeus (or Jupiter), other deities that were recognized and worshipped by the ancient Greeks and Romans were outside that order, and they recognized foreign deities as being just as real and worthy as their own ones.
Other Religions: This is probably the most "easy-going" religious option, since it recognizes all deities of being more or less equally worthy. They'll see the other religions as being simply cults to other deities that can be worshipped normally. Differing theistic world-views (i.e. monotheism, henotheism etc.) will be seen as the result of deities with a superiority complex; world-views that don't include deities "per se" (as atheism and pantheism) will be seen as sorely misguided.
Character Classes: Follow the default D&D guidelines, since this is the "default" D&D religion. Clerics and paladins (as well as most monks) will be devoted to one of the deities; druids and rangers subscribing to this world-view would worship a nature deity; and members of other classes might be drawn to some particular deity (like wizards to Boccob), or follow whatever deity their community favors.
Pantheon-based Polytheism
This alternate form of polytheism is more common among historical cultures, and differs from the previous one in that all deities join to form a pantheon, which usually has an organized hierarchy and a chief deity. Examples might include the Chinese, Egyptian, and even Greek pantheons (if you analyze just the 12 Olympians), and some might even interpret Catholicism as a form of polytheism, if the saints are seen as "lesser deities". In this kind of religion, worship is usually centered on the head deity, but also directed toward the other ones, who are considered to work in cooperation. There will be rituals dedicated to the different deities, depending on their area of influence; for example, the Harvest Feast would be dedicated to the agricultural god/goddess. There may or may not be orders or temples dedicated to individual deities, but even their members will recognize and worship the other deities, especially the pantheon's head.
Other Religions: The gods of other religions will usually be considered as lesser or unworthy gods, who don't belong to the "good" pantheon and don't relate to the "greatest" deity. Alternatively, they might be considered as different faces of the pantheon's deities, or lies set up by some evil or mischieving deity. Otherwise, the opinions about differing world-views are generally similar to the previous entry.
Character Classes: The way clerics work will depend on whether the religion has separate orders for different deities. If there are, then there will be clerics dedicated to those deities, but who still recognize the pantheon head as supreme. Otherwise (which is more common in actual societies), all clerics will be alike, worshipping all the deities at once, and will be like clerics of the same deity in D&D terms. Paladins, monks and druids who belong to one such religion will generally belong to specific orders, if there are any. Members of other classes will generally worship the whole pantheon, but some of them might be loosely associated with a deity's order.
Henotheism
This is a special kind of polytheism, since its adherents believe that there are several deities, but only one of them (or only a limited group of deities) is worthy of worship. The other ones might be considered as "lesser beings" who don't compare to the "true god"'s greatness, or as fakers who hardly have anything of divine in them, or maybe just morally unacceptable (which is often the case with a henotheistic religion based around a Good deity). One real-world example that springs to mind is early Judaism, which has Yahweh facing off against other deities and proving his might, although later Judaism should be considered as monotheistic.
Other Religions: As should be inferred from the previous paragraph, other deities are seen as being "lesser" or unworthy, and their followers are just poor sods that chose the wrong god (or goddess) to pray to. People who don't believe in individual deities, to a henotheist, are just more misguided than the rest.
Character Classes: All characters that subscribe to a henotheistic religion, regardless of class, will worship the deity that's considered as being the "right one". If the religion has both clerics and druids, an explanation as to how are there different ministers of the same god will be in order; maybe they belong to different orders that focus on different aspects of the same deity.
Dualism
This kind of religion focuses around a conflict, a contradiction, between two opposing principles. This is most commonly represented by a Good and an Evil deities, but it could also be Law vs. Chaos, Light vs. Darkness, Masculine vs. Feminine, some other opposition, or maybe a combination of several opposed aspects. Religion is based on a constant struggle against the enemy deity, and the world is seen as being split between the two sides - it's a war, and you have to take sides. If you don't, you'll end up unwittingly serving one of them. The most classical real-world example is Zoroastrianism, with the opposition between Ahura-Mazda and Ahriman, but modern religions commonly thought of as monotheistic (such as Christianity and Islam) can be seen as dualist, with God (or Allah) versus the Devil.
Other Religions: Everything belongs to either of the two sides of the divine war. Therefore, other religions might be seen as different ways to see one of the two contending deities, but are more commonly seen as lies set up by the "bad" or "enemy" deity. This also includes non-theistic philosophies, of course. Therefore, the members of other religions are generally seen as enemies, even though they don't actually realize it, and will generally be confronted or converted; some religions, however, may be seen as being in the "good" side, and their adherents will be respected and appreciated.
Character Classes: Similar to the previous entry, since all members of a given dualist religion follow the same deity. Alternatively, some character classes (for example, wizards or druids) might be seen as being in league with the "enemy", and will not be tolerated within this religion.
Monotheism
Members of monotheistic religions or philosophies believe that there is a single deity. This deity may manifest itself in several different ways, or be known by several different names, or maybe none of that - regardless, it's the same individual entity behind it all. Some modern interpretations of Judaism, Christianity or Islam may fit this scheme, although they often have a dualistic element; non-denominational philosophical theism would fit the bill more accurately, as well as "new-age" philosophies, although the latter often tend more toward pantheism.
Other Religions: As said before, some monotheists will believe that the single deity has several different forms and names; in that case, the different religions will be seen as just different ways to see the same entity. Otherwise, they will be seen as misguided and mistaken about the world, and their deities as being either imaginary or non-divine creatures. Non-theistic world-views will be seen as being simply plain wrong.
Character Classes: Since members of a monotheistic religion all worship the same deity, they work the same way as described under the two former entries as well. Additionally, it can be easier to introduce distinct religious orders (such as clerics and druids) if the religion is of the kind that admits different interpretations of the single deity.
Pantheism
This kind of world-view doesn't believe in individual, given deities, but only in an all-encompassing divine principle that is present in the whole Universe. For the pantheists, there is no difference between God (or whatever the name) and everything else, since everything is comprehended by the divine. Such religions often have a mystical leaning, with rituals, meditations, or other means to reach oneness with the divine and "lose" oneself in its infiniteness. One traditional example of pantheism is Taoism, with the cosmic Tao, composed by both Yin and Yang, being the all-encompassing divinity - but Hinduism might be considered as a form of pantheism as well, even though it looks like polytheism (see "Pseudo-Polytheism" below). Modern "new-age" religion is often pantheistic as well.
Other Religions: Pantheists often believe that the divinity can be seen or interpreted in several different ways, and therefore see the differing religions as just other ways to see the same principle, and the gods and goddesses as just different faces of the infinite "Divine". Alternatively, they might see the different "deities" as being fakers that cannot be actually called "divine", although they may be more "connected" to the divinity than mortal beings.
Character Classes: Pantheism is possibly the philosophy that fits druids the best; after all, if the divinity is everywhere in the world, there's nowhere better to find it than where the world expresses itself in the most splendid way, which is nature. Of course, pantheism may fit clerics as well, and those clerics will usually tend toward neutrality, since there's no personal principle behind their faith. Pantheistic paladins will stray from that tendency, however, generally because their philosophy incorporates a strict code of conduct. Pantheistic religions might possibly have distinct orders, which view the Divine in different ways, which is reflected by different divine-casting classes. Pantheistic members of other classes have no special considerations, especially due to the universality of this philosophy.
"Pseudo"-Polytheism
This is more of a "side note", or a distinct quirk that certain religions might possess. Some religions appear to have several distinct deities, while they are in fact different facets of the same deity or divine principle. The most notable example is probably Hinduism, whose many deities are considered as being simply distinct forms of Brahma, which, more than a deity, is considered as being an all-encompassing divine principle (making it more like pantheism). The "Holy Trinity" of Christianity might also be considered as a form of pseudo-polytheistic monotheism.
Other Religions: Since this kind of religion already has the "different faces of the same deity" principle, it has a tendency to accept different religions as being different ways to interpret the same truth; that is not always the case, however, and then it will be as per the "base" religious world-view.
Character Classes: Religions of this kind often have distinct orders or temples dedicated to the different aspects of the deity, and those might be different kinds of cleric or even distinct character classes. Otherwise, treat as the "base" religious world-view.
Animism
This is a kind of religion that's common among "primitive" people, even though there are faiths more or less resembling it in more "civilized" cultures. Animists believe that everything has a soul, not only that, but also that there are several spirits that wander around and can be contacted for favors, blessings or informations. They usually contact either nature spirits, which tend to be animal-like or maybe plant-like, or ancestral spirits of the tribe's forefathers, especially the more important and memorable ones, who stay behind to help out their kindred. Real-world examples are difficult to list, but include several tribal peoples, especially ones in Africa and the Northern polar region; Japanese Shintoism (as well as some traditional Chinese beliefs) may be seen as a form of shamanism as well.
Other Religions: Animists will usually see the gods of other religions as simply spirits, and just find the ultimate devotion of clerics and paladins a bit strange, since the animist relationship with spirits is much more practical and almost business-like. Pantheists, atheists etc. will usually be viewed with bafflement, for their "strange" beliefs.
Character Classes: Animist "shamans" would ideally use their own character class, and indeed, I have made a homebrewed shaman class to use in my own campaign (where shamanism is quite important). If you're not up to do that, the druid can be used as a reasonable approximation, especially if nature spirits tend to be contacted; those animists who focus exclusively on the spirits of deceased people could very well be portrayed as clerics, and would be a kind of "psychic". Other classes can be used normally to portray characters of animistic societies (in my aforementioned campaign, there's a paladin who's devoted to the spirit of the Puma), but understandably, Barbarians are the most fitting characters for such cultures. However, sorcerers can be a nice choice for a "witch doctor" kind of shaman, especially when coupled with a divine-casting class.
Totemism
This, alongside with shamanism, is the other kind of religion that's common among tribal societies. In fact, totemism is associated with a specific kind of society - the ones based on clans, which join together to compose larger groups such as tribes. It's found chiefly among North-American natives (where the name came from), Australian aborigines, and some South-American native peoples as well. This kind of religion assigns a totem to each clan, which usually is represented by some animal or plant, and stands for a kind of spiritual/supernatural being which the clan is faithful to. It is not quite the same as animism, since there's a permanent bond with the totem rather than the convenience-based "contracts" shamans make with spirits; it's not quite the same as theistic religions either, however, since the totem is considered as being "one" with its correspondent clan, even synonymous with it, rather than above it as a deity. This world-view often divides the world among these totems, mirroring the tribe'sdivision between the clans.
Other Religions: Totemists will often see other deities as other totems that represent the respective societies that worship them, and will have trouble understanding societies that worship several deities at once, with no geographical division. The way deities are viewed in other societies will also sound strange to them.
Character Classes: Totemist priests would be rather depicted as druids, with a tendency to "Wildshape" almost exclusively into their totem animal. Clerics aren't terribly fitting for this kind of religion. Just like with animism, other character classes may be present, but the barbarian is the most typical class for this kind of society.
Deism, Agnosticism, Atheism
Although these three philosophies are quite different from each other, I have banded them together because their practical implications in a campaign are surprisingly similar. So I'll describe each of them separately, and the "Other Religions" and "Character Classes" sections will be unified for the three.
Deism: Also known as the "Absentee Landlord" theory, this philosophy postulates the existence of a supreme deity that created the Universe, and then left it alone and didn't do anything else since then. There may be several different philosophical explanations for this behavior, but what matters is that Deists don't believe that any divine entity is at work or responsible for anything that happens in the world, regardless of the fact that there originally was one. It has become common since the "Enlightenment" period, among scientists who believe that an intelligent creature must have been at the origin of the Universe, but that everything else is explained by science.
Atheism: The adherents of this philosophy don't believe that there's any deity at all. There's a philosophical distinction between those who actually postulate that no deity exists, and those that simply don't see any reason for believing there are any; but this doesn't matter in D&D terms, what matters is that atheists don't believe in any deities or divinity. There are several atheists all over the world, who usually don't form any unified "religion"; however, a curious example of a more or less atheistic religion is Buddhism, or at least the most "orthodox" version, which believes that everything in the world is an illusion and not real, including the gods.
Agnosticism: This is a philosophical current that postulates that, whether there is any God(s) or not, nobody can know. It can range from simply "I don't know right now, but I could decide given enough evidence" to "It is absolutely impossible for anyone, for such-and-such reasons, to know whether is there a God or not". Regardless, what matters is that it has no set position on the existence of deities. Understandably, agnostics in a D&D world would be skeptical of divine casters for some given reason, otherwise they'd have all the proof of deities they needed... so therefore they'd tend not to believe that deities are behind that. In the real world, agnosticism is a philosophy that, like atheism, has no unified community, but is even more spread out throughout the world.
Other Religions: The most important facet of these three world-views is that they're usually philosophies that are held by individual thinkers, not having organized religions of any kind, although this isn't necessarily true (again, Buddhism comes to mind). Because of the nature of their beliefs, their members will generally see the deities of other religions as being simply and purely false - either powerful creatures that aren't divine like they claim, or just non-existent imaginary beings. Otherwise, they will generally not have much against other religions, aside from philosophical differences.
Character Classes: Surprisingly, clerics and other divine casters aren't really unthinkable for such "god-less" philosophies. You may have an organized religion, with its own philosophy, etc. that fits into one of these options, like Buddhism, but that needn't even be the case. Non-theist divine casters may get their powers from a certain philosophy that many people share, or out of "cosmic awareness" or "harmony with the cosmos". None of these requires the existence of deities, so they would work normally with the aforementioned philosophies. Of course, other character classes may be used normally who subscribe to one of these philosophies.
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That's it... remember that this isn't a comprehensive guide, the details of each religion are still up to you. Besides, keep in mind that the categories above aren't set in stone - there may be religions that join elements from several categories, and a real-world example would be Christianity, which joins monotheism, "pseudo"-polytheism, dualism, maybe also henotheism, and even a form of pantheon-based polytheism in the case of Catholicism. So feel free to put similarly "mixed" religions on your campaign, which will only contribute to make it more interesting. If you can come up with some religion that doesn't fit at all in any of these categories, great! You've shown a lot of creativity, and your campaign world will benefit from it. Just go ahead and don't be afraid to make your campaign's religions different from all the players' expectations...