D&D has too many cultists in their adventures. Let’s give them other ideas to replace cultists
I'm going to give an entirely different take.
D&D religions is vastly too influenced by Catholicism, despite lacking any of the cosmology which makes Catholicism reasonable. That is to say, cults in D&D are either always inspired by the Catholic Church or else are always inspired by heretical opposition to the catholic church of some sort.
Since the vast majority of D&D settings are pluralistic, polytheistic, and animistic none of that makes the slightest bit of sense. In fact, D&D settings tend to have
vastly too few cults. The only cults that show up are occult anti-religions. In fact, there ought not be any 'churches' in the setting (and I consider the use of the term mildly offensive): it ought to be all cults. All religious practice ought to be cults, and based on the prevalence of activity deities or at the very least their agents (clerics), such practice ought to be fundamental and pervasive.
When imaging what a cult or religious practice looks like in a D&D setting, it's best to avoid all ideas of Western organized religion, and instead consider something more like a fraternal organization like the Masons, the Shriners, or The Elks.
The following things ought to be true based on the assumptions of most D&D settings:
a) Almost everyone tends to have multiple patron deities, or at the very least feels quite free to worship broadly regardless of the alignment of the deity, if only to propitiate said deity.
b) Most worship ought to be communal rather than private. Most worship probably isn't particular sincere or pious as we understand the term, any more than most people pay taxes out of sincere love of their government. Most worship would be perceived as transactional. I'll do this, now don't murder me and maybe I'll get some benefit out of it.
c) Most people at least nominally belong to multiple cults, in the some way most people belong to multiple organizations.
d) All organizations are in some sense cults. So if you have say a Gym Membership, that would in setting involve membership in some sort of cult of athletics or strength, and have rituals and rites, priests and holy days associated with it. And on the holy day, people would go to the gym, lift weights, and what have you as an act of worship. Likewise, if you are a member of a Discount Club, or in a Professional Organization, or in a Trade Union, all of those things would be membership in cults. And likewise there would be probably Neighborhood Cults, and Homeowner Association cults, and heck if you had running water or sanitation that would probably involve membership in a cult. You would be paying the cult for the service of clean water. The folks that pick up the trash in the morning are a cult one way or the other.
e) There would be absolutely no such thing as separation of church and state, and certainly not seen to the great extent seen in the Middle Ages where you had a secular head of state and a parallel religious heirarchy which only partially overlapped the secular organization. No, justice would be dispensed by a cult, either formally as a recognized office and role in government or informally because the Police Union was a cult. The Judge might formally be part of a cult with the recognized role of judging, or defacto in control of the judicial system because Judges tended to have membership in a cult and anyone not in the cult was actively undermined by the judges that were in the cult. The army itself would be a cult. The king himself was the head of a cult that corresponded to the national identity.
f) Many cults would involve worship of more than one deity, in an effort to garner the favor from anyone or anything relevant.
g) As just a practical matter all organizations would tend to have a ritual and religious component for the same reasons that all parties tend to have a cleric. It's just not practical to try to go it alone without the benefit of some sort of divine favor and blessing.
Viewed this way, there is absolutely nothing you can replace a cult with.
Organized Crime? That's a cult with a massive religious component to it. Consider the Yakuza, Mexican Gangs, or even the Mafia just in the real world.
Pirates? What is that but organized crime? Every boat is a cult headquarters and no one is going to be sailing anywhere without propitiating various powers of wind and wave. Ditto every bandit group, which is just organized crime in a rural setting.
The anti-social cults might still exist, forbidden as part of daily life and attracting the desperate or the insane. But there would be a lot of religious strife of a much more subtle sort. And there would also likely be vast stretches of the world where morality wasn't neat and tidy and convenient, and players wanting to play Good would find challenges beyond the usual puppy chewing nihilistic End of the World lunatics.
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