I suppose I asked for it. Not directly, to be sure, but in the shape of my rhetoric. I'd ask, all confounded, if the rules to AD&D really meant that the Thief class really had to be thieves, and that would create the implication that perhaps a character's class in the rules layer didn't have to map directly to their job in the fiction layer. Maybe a Thief could really be a detective, or perhaps a Fighter could steal things and thereby become a thief. Maybe a Priest doesn't need to be a priest.
And then there's Earth, Air, Fire, and Water, by Shane Lacy Hensley. It tells us all about the Priest class in the Dark Sun rules, but almost nothing about the religion of the Tyr region. In terms of lore, the Priests of Athas are actually closer to later editions' Warlocks than they are priests - they make pacts with beings from the elemental planes and in return they gain appropriate elemental powers. These elemental beings are explicitly not gods (in fact, the book scoffs at the very idea - "Many earth shamans are known to worship the 'God of the Volcano,' for instance. That deity does not exist, but entire cultures have arisen based on such false beliefs"), but the book never makes clear why that matters. Okay, so what you're worshipping isn't a "god," but somehow "ineffable spirit of the land" is less dignified.
The introduction poses the question, "In a world with no gods, what do priests serve," and that betrays to me a certain bias. As an atheist, it feels a little redundant to me, but I'm trying to mean that in a spirit of genuine inquisitiveness, rather than snark. The truncated question "what do priests serve" is an important one for building a satisfying setting. My atheist answer is "the community or the culture," but I'm not sure any version of Dungeons and Dragons, even the modern ones, is on that same page.
Community is thin on the ground here. There's bits about Earth Priests teaching sustainable agriculture and Water Priests purifying water, but mostly the text treats priests as powerful individuals and not part of a cultural tradition. I get the feeling that the gods being real has been doing a lot of heavy lifting for D&D's depiction of religion thus far. You can infer anything you need to know about Pelor's doctrine, or his church, or his ecclesiastical organization by drawing on your knowledge of the nature of Pelor. Priests are like the god's assistants, and so they just sort of act in a way that furthers his goals.
It's an approach that definitely shows its limitations when you remove the god from the equation. The priests of Athas are still vaguely assisting their patrons' agendas, but now the patrons are primordial elemental forces, and thus the agendas are super-simple. They really just want to exist more. Fire priests go around encouraging the growth of forests and cities, because both are combustible. Water priests get mad at people wasting water. There's not a lot of room for ordinary believers (hell, there's nothing much to actually believe).
I think Athas definitely feels poorer for the elemental priests not having a coherent theology. We don't learn anything about weddings and only a little about funerals (Fire priests encourage cremation, even though the elements as a whole benefit most from burials), no holidays, festivals, moral lessons, or theories about the afterlife. Just encouraging the existence of elements, all day long.
It wouldn't be so bad, Priests being Warlocks, except that it exacerbates one of the setting's biggest problems - the existence of too many types of magic. Wizard magic is hated because it is powered by life force and greedy mages are responsible for the desolation of Athas. Fine. Fair enough. But then you have other things which might look like wizard magic, and they're not hated. You can draw a distinction between the internal power source of psionics and the external power source of magic, and that's meaningful on the level of the setting's physics, but . . . can the average person tell the difference? Maybe they can. Maybe the ubiquity of psionic powers gives people a sense for when psionics are used. Wizard magic feels unnatural, and so it is shunned. It helps that in AD&D, magic really was broadly more powerful than psionics, and thus a realistic temptation.
But actually, there are two types of wizard magic. Defilers destroy life to gain their powers, but Preservers don't. They draw life energy from the world, but in a way that leaves the natural balance intact. And in this case, you don't want the average person to be able to tell the difference. That's how you get an underground society of Preservers like The Veiled Alliance. Practically, that means you can't just have people look for the piles of ash and say, "hey, that's the kind of magic we hate." Casting of any kind is suspect.
Which brings us to Priest magic. It too is external magic, and while you could draw a distinction between Preservers "draw from the world, but no more than it can handle" and priests "draw from the elemental planes which compromise the world," the difference is likely to be lost on a layman. Add in the fact that Priest spells are cast in a very similar manner as Wizard spells (with Verbal, Somatic, and Material components, from a list memorized each day) and the natural question to ask is - why aren't Priests treated with the same suspicion as Defilers? That they're doing different things is only obvious to us because we can see their character sheets.
Earth, Air, Fire, and Water gives priests a lot more to do, and offers helpful suggestions for roleplaying them, but in the process makes them more redundant with Preservers than they've ever been.
Ukss Contribution: Curse of the Black Sands. It's a spell that makes the target leave behind oily black footprints whenever they step on bare earth. Useful, but also just a neat visual.