The variation within the mechanics usually manifests itself in the various domains attributed to a particular deity. For instance, Forgotten Realms includes Tyr as a god of justice, covering the Order and War domains (5th Edition has simplified this somewhat from previous editions). So a player choosing a Cleric or Paladin of Tyr has a few powers related to these domains. If they meet followers of Tyr in game, they'll know they're forthright and honest. Probably very interested in justice and enforcement of the law. This is how it goes for every deity in the system. There are some power tweak for players and there are some identifying characteristics for NPCs.
Now, a clever DM will, of course, accentuate some traits and mask others. Maybe, in the case of Tyr, the local temple leader has an idea of justice that's more the letter than the spirit of the law and many people are getting thrown in jail on frivolous charges. It's turned neighbor vs. neighbor and either the players have to navigate this problem or solve it. Maybe the head church of Tyr sends the players out to find out why a paladin of theirs is laying waste to the countryside. Turns out he's trying to bring a person or monster to justice and doesn't care about he collateral damage. Again, the players can help or hinder depending on their read of the situation.
This is about as far as it ever goes, however, with regard to this religious expression in DnD. The next step, I think, is to manifest heterodox religious practices. This doesn't mean perversions of existing religions, which is another common trope - the local priest of Pelor has been seduced by the dark and runs terrible rituals involving a goat man and human sacrifice. Think Martin Luther or Jesus or various flavors of Baptist, Buddhist, Judism, or Islam. Wars have broken out over issues that might seem minor to the outsider but represent an attack on the core orthodoxy of the existing church. Of course there are probably campaigns out there that delve deeply into these issues and if you run one of those, I'd be interested to hear what gave rise to the idea and what you've done in the world.
The genesis for this post is probably this tweet coupled with a re-read of Anathem by Stephenson:
https://twitter.com/misspenart/status/1119630786557349888
I've been thinking about how to increase representation in my games anyway and this tweet got me thinking really hard about how my NPCs were usually drab dudes with different voices. There's no reason for me to not to replace the local mayor/alderman with a woman, except that I didn't really think about it before. Likewise, I always thought I played interesting characters but they're generally dwarves of one stripe or another who like to hit things. Out of this tweet, I dreamed up a one legged rogue. He's got a pretty fool proof disguise when he's in a big city - few people spend time on the blind beggar leaning against the building looking like he's got the plague. I figured he could hide in plain site as long as he's in an urban area. Out in the woods, he'd probably be a bit more of a liability, which would be kind of fun to role-play.
The central theme of this line of thought is that it's a fantasy world. There are all sorts of possibilities and to allow them to be unexplored is a shame. So instead of allowing our characters and their methods of worship to be these unchanging monoliths, I thought it would be fun to loosen them up a little.
Most of what I've schemed up represents small changes to existing orthodoxies. These aren't very daring changes to the existing gods but I felt it best to start with small changes and build up from there. Here's what I have imagined so far:
A matrilineal Dwarven hold that revolves around a matriarchal sect that believes Moradin and Berronar are two aspects of the same deity. The hearth and the forge are two sides of the same coin. Most of the master crafters are women and child rearing plays a huge part in the community.
An interpretation of Lathander that involves some texts the main church views as apocryphal. Lathander has always been very strict on the subject of undead but this break-away church holds the view that the reason this is so is because the soul of the undead becomes trapped in the material plane forever if the body is destroyed without certain precautions.
A view of Lloth that's much more complicated based on which of her family members one believes took Lloth's place after the great silence (which is when she died/was killed/transcended to something more powerful).
Worshippers of Akadi who interpret her teachings as not traveling physically, but mentally or spiritually. They work to learn everything possible and study about how astrally project into other planes of existence.
There are many more that can be created. I'm not the most familiar with the current 5e Pantheon so I'm sure others can create much more imaginative diversions from the monolithic orthodoxies that are represented in the Dungeons and Dragons of today.
