I’ve been reading Playing the World, Jon Peterson’s scholarly history of the origins of Dungeons & Dragons. Peterson elucidates the circumstances around the wizard/cleric distinction well, explaining that such a distinction had very little basis in fantasy literature (and none in Chainmail, Gygax’s predecessor to the D&D ruleset) but instead appears to come from Gygax’s religious background & various complaints about the absence of religion in medieval & fantasy wargaming. As a result, while magic users are fully irreligious, we have a separate class that is essentially a specialist magic user whose domain of abilities is based on a credulous reading of christian miracle-work — along with a dynamic involving power level being limited by a fall from grace. This is wholly at odds with the essentially pagan attitude elsewhere in the extruded fantasy product tradition that D&D in many ways codified: a universe where a thief can be lawful good is not in any way a christian universe, even in terms of the secularized pseudo-christian morality that binds clerics. Instead, since we’re talking magic systems, we might look at historical occult traditions and classifications: specifically, the thelemic conception of left-hand versus right-hand path.
Talking to magick practitioners about the left hand and right hand paths is like talking politics at a family gathering: it’s a recipe for broken hearts. For the sake of this essay, I’m going to use a strict and simplified division: left-hand magic is performed using the abilities of the magic practitioner directly, and comes down to a set of rules and mechanics; right-hand magic, on the other hand, relies upon no special abilities in the practitioner, but instead a knowledge of means by which the practitioner can convince supernatural entities to perform tasks. This is certainly not the only way to interpret left-hand vs right-hand traditions, but the major alternative (which applies a moral gloss, considering left-hand to be synonymous with black magic) is less useful for the purpose of role playing systems & ultimately redundant; I would furthermore claim that conflating the two (as many traditions do) is at best an indication of anti-secular bias.
Untangling left-hand and right-hand mechanisms in historical occult practices is difficult, in part because non-secular traditions perform theological gymnastics to reclassify seemingly left-hand practices as right-hand practices, thereby avoiding anti-left-hand bias. This is particularly common in monothesistic contexts: the use of kaballistic formulae whereby the magician manipulates letters or numbers representing the state of the world in order to manipulate the world appears fairly left-hand, but by identifying the material world with an aspect of the divine and invoking predestination, we can treat it as a form of prayer — that most representative form of right-hand magic. For the sake of this essay I would like to avoid these kinds of tricks entirely, and consider only those magical techniques that directly interact with a supernatural entity as right-hand.
And so, what does this give us? It gives us a richer domain for the cleric, reallocating some of what would otherwise be the domain of wizards. In a clearly polytheistic context, the low-level cleric also takes on some aspects of bards, being capable of granting boons to particular attributes by prayer or sacrifice to gods associated with those attributes. Contracts with gods, as in voodoo, are the domain of clerics; control of elementals, summoning of & contract with demons, and invocation and evocation of any non-humanoid ‘monster’ likewise. Rather than needing to keep up a kind of pseudo-christian morality, a cleric would need to merely act in a way in line with the gods he or she actually interacts with. (Many polytheistic pantheons contain a god of communication who acts as a gatekeeper: Legba in voodoo, Ganesha in hinduism; any cleric would need to act in a way that pleases the gatekeeper deity, or risk losing access to the whole pantheon, but what this means is very different between pantheons: what is fine with Mercury is unlikely to be acceptable to Ganesha.)
This would require moving many of the healer & support roles to another class; after all, the cleric redefined in this way would be significantly more powerful as an offensive class at higher levels and would not have special healing abilities at lower levels.