Religion in Dungeons & Dragons has always been something of a conundrum for me. While on the one hand, the system has elements that are built in that assume a polytheistic arrangement, the base elements of the system are heavily weighted in favor of monotheism.
Way back in the mists of time when role-playing games were first being developed, the games were more or less just an adjunct to wargames. Some versions of early role-playing games were nothing more than a side element of a larger wargame - the main wargame would involve a medieval army laying siege to a fortress, and the "role-playing" element took the form of the participants taking the roles of heroes from the besieging army infiltrating into the fortress via the castle sewers. This resulted in the idea that players would play a single character, and also more or less originated dungeon crawling.
But the important facet of the game was that even though each player was responsible for a unit consisting of a single individual, these were not really characters. They were military units in a wargame. and they all had roles to fill. Fighters were for fighting things. Wizards were for blasting things. And clerics were for healing. Thieves were added later, as were all of the other character classes. There was no real thought given to where clerics derived their powers. They were a wargame unit with a certain array of abilities. They were not as good at fighting as a fighter. They were not good at blasting like a wizard. And they could heal. That was pretty much the sum total of thought put into the game design element. And that works perfectly fine for a wargame.
Eventually, the game that would become Dungeons & Dragons reduced the focus of the wargame element and made the role-playing more important. And note that I said "reduced", not "left behind". Because if one goes and looks at the old first edition Dungeons & Dragons books, the tendrils of its wargame association weave throughout the rules. Distances are expressed in inches. Time passes in the game in segments, rounds, and turns. The game directs players to put together a "marching order", and have one player be the conduit of information to the DM. Characters are assumed to aspire to build a fortress and draw together an army of followers that they could put into the field. And so on and so forth. And the basic character classes reflect this. Which meant that clerics still held to their role as healing batteries for the other players.
In these early years of Dungeons & Dragons, that was pretty much all the thought that was given to religion. Clerics came in two flavors: good and evil. Good clerics refused to shed blood (using bludgeoning weapons) based upon the fact that Bishop Odo (the brother of William the Conqueror) used a club as a weapon. Good clerics could heal others and had a collection of magic that mostly bolstered their allies plus a grab bag of spells drawn from various effects found in places like the Bible, medieval tradition, and a couple other places. Evil clerics had magic that was mostly just the magic of good clerics reversed. For some reason, evil clerics also retained the prohibition on using non-bludgeoning weapons as well. In effect, the game assumed something more or less akin to a pseudo-Catholic medieval church transported to whatever fantasy land the adventures were set in, and didn't really clarify things much further.
And when one looks at old published adventures and game materials from that era, it is clear that this fairly simple paradigm of religion in D&D reigned supreme. When the paladin class was introduced into the mix, it fit alongside the medieval pseudo-Catholic cleric in the role of a crusading zealot out to smite unbelievers and evil-doers. The fact that evil clerics were the exact opposite in many ways from good clerics also explains the constant drumbeat of players wanting to know when the rules for an "anti-paladin" would be put forward. After all, if you have what amounts to a cleric and anti-cleric class, then you should have a paladin and anti-paladin. For the relatively wargamish style of dungeon delving adventures that dominated the early years of role-playing in Dungeons & Dragons this system worked perfectly well. When the druid class was introduced, it was placed in the "neutral" position, but to do so, a whole set of new mechanics had to be created, because simply making a "neutral" cleric would have been difficult under the then presiding cleric paradigm. When you have a "pro" and an "anti" as your established design, a middle ground is hard to make work.
In 1980, six years after the first print version of the game was released commercially, TSR published the Deities & Demigods game book. The book itself was kind of mediocre, presenting the deities in its pages more or less as supermonsters with not much detail concerning the religious practices that should be tied to them. (Plus, it had some legal problems as the book included material from the Cthulhu, Melnibone, and Newhon settings without getting appropriate permissions from the owners of those intellectual properties). But that seems more or less predictable. Clerics were basically undifferentiated mechanically no matter what deity they chose to follow. There was really no reason to give the faiths of the various deities more detail because clerics were essentially all drawn from the same two flavors: good and evil.
But the introduction of such a plethora of deities and the development of players more interested in more in-depth (or sometimes "more realistic") role-playing resulted in an internal struggle in the dynamics of the game. Confronted with a fairly rigid alignment system, a fairly two-dimensional cleric class, and a system that more or less tacitly assumed the kind of monotheism that prevailed in Western Europe during the middle-ages, game writers, dungeon masters, and players struggled to make games work with a collection of polytheistic pantheons. And those results were often somewhat hilarious. I have an old adventure that was published in Dragon magazine during the 1980s in which the adventure writer has to explain that clerics devoted to the Celtic god of death Arawn can use daggers for ritual sacrifices, and then goes on to explain that this doesn't change the fact that they can only use bludgeoning weapons in combat. And this sort of silly explanation is the direct result of the pseudo-Catholic roots of the cleric class which made it ill-suited to provide a framework for a variety of polytheistic divinities. In another development, the same module also saw the introduction of the "huntsman" class, which was basically just a repackaging of the ranger class with the sole alteration being that huntsmen were required to be evil.
And the pseudo-Catholic design of the cleric class (and incidentally, the paladin class) fits quite poorly with a polytheistic system. While polytheistic religions in our history did have concepts like heresy, the idea of an infidel is somewhat foreign to them. In a polytheist's mind, all of the gods are real, even the ones that are "evil", and mere mortals don't strive against the gods. For an ancient Greek, Zeus, Hera, Poseidon, Hermes, and Ares were all quite real, and all had their sphere of concern. A sailor might look to Hera to ensure his wife had a good delivery of her pregnancy, and to Hermes to make sure the physician tending to his ailing brother was skilled, and to Poseidon to ensure he had a safe sea voyage. But he would not look down upon a smith because he had a shrine to Hephaestus in his smithy, or be annoyed that his neighbor offered prayers to Demeter in the hopes that her garden would be bountiful. All of the gods had their sphere of influence, and he might need to call upon one of them for aid at some point. Even foreign gods were still gods, and still worthy of honor. A Greek sailing to Egypt didn't think that its inhabitants were somehow in the wrong for venerating Horus, Anubis, and Isis. In fact, he might translate their gods into terms he was familiar with, or bring an idol of one of the Egyptian gods home so that his countrymen could be sure to offer prayers to this divine being. The Romans made it a practice to try to identify all foreign gods with their own pantheon, equating the various divinities of other peoples with their own Jupiter, Mars, Mercury, and so on. In short, the cleric/anti-cleric design of the classic Dungeons & Dragons setting was poorly suited to reflect a polytheistic world.
But polytheism became the standard model for Dungeons & Dragons campaigns, resulting in a collection of mostly ad hoc rules additions to make clerics more individually attuned to their particular deities. This seems to have first been widely implemented in the Forgotten Realms campaign setting, but it was then ensconced in the second edition Dungeons & Dragons game via the somewhat modest mechanic of clerical "spheres", which served to customize a cleric's spell selection to a certain extent. The third edition Dungeons & Dragons game uses a system of having clerics select "domains" which give them bonus spells to choose from plus some sort of unique ability. But the system is still, at its roots, built on monotheistic assumptions, and a whole host of spells, class abilities, and other game effects are tightly tied to the alignment system that flows from those assumptions.
Without eliminating or massively rewriting numerous character classes and discarding the alignment system (and consequently discarding the large chunks of the game that are built upon the alignment system) it is difficult to create a religious system in a Dungeons & Dragons campaign setting that is not monotheistic in character (or, more accurately, dualistic). But to make the system work as it is structured now, one must provide a collection of divinities to attach clerical domains to or have the somewhat game-breaking set up that allows a cleric to choose almost any pair of domains that they want. Plus, losing polytheism, unless well handled, would rob the game of a fair amount of fun.
So what is a campaign designer to do? When confronted with a game that is filled with mechanics that point to monotheism sometimes and polytheism in other places, the solution that I have usually come to is to create a religious system that is a hybrid of monotheism and polytheism. I have taken a couple different tacks with this sort of arrangement. In one, I established that there was a single, distant deity but that the inhabitants of the campaign interacted with that deity by means of a collection of intermediaries such as archangels and saints. Each cleric would attach themselves to a particular intermediary. All of them were opposed by a cadre of rebellious fallen angels, who evil cults revered. I'm not going to use that exact system for the Realm campaign, because I want to use divine powers that are influenced by the Celtic and Norse mythologies, but I'll probably implement something that follows similar lines of reasoning.
Home Campaign Design Theory