No matter how much detail a game entity such as a character, place, or even a deity has… it will only get interesting when it connects to other things.
When it comes to deities, a deity standing alone is unlikely to be of any real interest. The connections between deities, and between deities and the rest of the world, and between deities and their followers… that’s where the interesting bits happen.
I’ve already set up one set of divine factions, and explored a second way of building factions. I think I like the second method better, so I’m going to explore that a little… later. For now I’m focusing on the mortal connections.
It seems many roleplaying games treat mortal followers of deities as worshipping a single deity. In our world historically, more often a culture would follow a pantheon but people would tend to make more offerings and give more prayers to specific deities relevant to their needs. A warrior would probably give most of his attention to a war god, but might well make offerings to the god of the roads (i.e. Travel domain) before setting out on a campaign — yes, the god of war might guide your sword, but the god of travel could keep you from getting lost, or lead you to better places to camp.
What does this look like?
I imagine there could be several levels of interest.
Most people will at least acknowledge the pantheon, or their culturally-related part of it. In a world where deities are real, rejecting their existence is a bad idea (possibly worse than actively opposing them! The Wolf God probably understands quite well why shepherds might fight off or even kill his followers and not take it personally, but denying the Wolf-God’s existence could well cause that existence to be made painfully evident). These people likely know of at least some of the deities and take part in rituals, but aren’t otherwise strongly connected to any particular deity.
Some people will have a pressing reason to follow one deity more than others. These are not necessarily ordained (i.e. not clerics or paladins or the like) but most of their thoughts of the divine are still toward their focal deity. They might be devoted followers, having been inducted in a ceremony and wearing symbols of their faith (such as many Christians wear a cross today). This often will be because the deity is perceived as aligning more to the follower’s needs than other deities would. Probably still no actual divine power involved (though that doesn’t stop the follower from offering prayers and sacrifices), but there could be greater support from the church.
An even smaller number would actually be ordained: formal members of the religion. Unlike the others they would have obligations to their faith. This is where you would find most clerics and paladins and the like (who actually gain power from their deity), but many in this group likely are ‘NPC classes’ (experts and commoners). In a pantheon-based religion I’m going to say that the ordained likely follow one deity most closely and work best with followers of other deities in the divine faction, but still will acknowledge other deities in the pantheon and their followers.
I’m not going to go into depth here, but I have seen (and have been adapting) rules for ‘affiliations’ from previous editions that should work pretty well.
In short, a character can develop ‘favor’ by meeting certain criteria and actively supporting the goals of the affiliation. This favor can be used to draw on the affiliation, and as certain thresholds are met the character may be able to advance within the organization. The rules I’ve seen so far look like these advancements happen more or less automatically (i.e. kill enough goblins and you’ll become guild master of the Goblin-Slaying Guild) but I feel there should be more to it than a big bag of goblin ears.
I have in mind that in addition to the ‘simple points-driven element’ there could be specific criteria that must be met. Even something as mundane as a guild might have a mandatory apprenticeship and/or journeyman period, require approval or even sponsorship from an existing member in good standing, submission of a masterpiece for examination by the guild (and possibly sale by the guild, in lieu of membership fee by the new master).
For holy orders I can see how there are certain baseline commitments (prayers, rituals, etc.) that could be expected. There can also be specific requirements for advancement (holy oaths and commitments especially) that might not even be considered unless the character measures up. That is, you have to exhibit enough merit to the order before even being invited to advance, and then to advance you still need to take the steps to do so. There are likely common elements to this from faith to faith, but the details probably differ based on the portfolios: advancing in the Church of the Healer might require that you save a life, while the Church of Death might be rather the opposite.
Though I can imagine that the Church of the Healer might require also demonstrated knowledge of end of life care… it’s not all black and white here.