I've been thinking about this for a while. I've wanted temple districts that are a mass of temples to various gods, godlings and deities, as you might find in some sword and sorcery worlds. But at first sight it seemed at odds with the renaissance technology, which, in my mind at least, brings to mind large organized religions.
But I've thought a bit about it, and I think I've come up with a solution that might work. It starts off with five assumptions.
1) Gods are real. The gods often communicate with their worshipers, and there can be no dispute to the fact that they are real.
2) The more worship, the more power. So most deities (and their priesthood) will want to have as large a worshiper-base as possible. Also, worshipers that worship often are worth more than worshipers that worship less often. A ceremony is good, a prayer is okay, an oath is better than nothing.
3) The increase in power is not directly proportional to the amount of new worshipers. Power increases slower than the amount of worshipers. This means that the more worshipers, the less power there will be to each worshiper.
4) There are many gods. If one falls, another is ready to take his place. There are a multitude of unworshiped godlings trying to pry worshipers from the established gods.
5) Worship is not exclusive.A worshiper can worship any amount of gods, though of course there will be less worship to each god.
With these four assumptions in mind, I think of three primary phases in the life of a religion.
The Cults: There is the greatest amount of these. They belong to godlings who have managed to gather a small worshiper-base. The priests have decent amounts of power at their disposal, and can help every one of the cults members if needed. In the smallest of the cults there is enough power that every member can be a divine caster. Cultists have an intimate relationship with their god, often being able to hear him speak from his idol, or at the very least being able to speak to priests which will be able to speak directly to the god. Cultists often focus most of their worship on their chosen god, sending only occasional, minor prayers to major deities. If the cultists devote large amount of time to worship in the form of ceremonies and everyday observance, the resulting power to worshiper ratio may be considerable.
The Pretender Churches: These are cults that are growing too big for the intimacy of the cults. A secondary or even tertiary layer of priests develop which can't communicate directly with the god. The power to worshiper ration decreases, and though there is more power in total, worshipers may start to seek out smaller cults that can better provide for their needs. Most churches fail at this point, as the god gets a reputation for being capricious or isolated.
Some churches manage to remain in this state indefinitely, by taking up a niche where worshipers will rarely seek out aid, but more often remember to send out a prayer. These usually try to spread to as many worshipers as possible, given the smaller amount worship that each worshiper provides. Among these churches some manage to grow large enough, and old enough that they become established churches.
The Established Churches: These churches have grown large enough to become part of everyday life. They have become known for their niche, and people will think to send them a prayer when encountering that niche, whether it is childbirth, battle, strength, or smithing. Their power derives from two, and sometimes three groups.
The first group is the incidental worshipers. They have little relation with the deity, but will send a short prayer, or swear an oath in its name. They are legion, and thus still provide a significant amount of power, despite their lack of consistent worship. Incidental worshipers may be aligned to another deity, or they may not be aligned to any deity, the closest that the world comes to an atheist.
The second group is the priesthood. Incidental worshipers will occasionally donate money to the church, and with enough donations a permanent staff of priests can be established, dedicated to worship of the deity. The hierarchy is likely to be deep, and while the high priesthood will wield significant power, some minor priests may wield less than a cultist. Some churches have dedicated monasteries, where the monks have no divine power, but spend their entire time worshiping, fueling the deity. Some monasteries or priesthoods may run various business schemes, thus giving the church even more money, with which to have more priests and monks.
The third group, that some churches have, are the consistent worshipers. These are everyday people who will still spend time to visit church and participate in ceremonies. They also provide a power boost, and is a good pool to recruit priesthood from and with which to inspire incidental worshipers. Consistent worshipers provide power, but they also drain power, as suitable miracles need to be performed to keep them in the church.
The priesthood and the consistent worshipers are both strongly aligned to the church, thinking of the deity as their one god. They may send the occasional incidental prayer other deities, but dual- or polyworship is not done, unless it is part of the churches doctrine.
Now, all this is background information. Neither the world's populace or the players will be really aware of these simple rules, despite the results being all around them. The primary reason are the churches themselves. Most cults and all churches develop some amount of doctrine, which includes their view on the world. Some cults and churches will claim that their god is the only true god or the only just god or simply the strongest god. Established churches have a wide tendency to purport that the gods of the established churches are different from the minor gods, and that they have always been there. The rising and falling of cults compared to the permanency of established churches gives a sense of a clear separation between deities and the false gods/demons. Every church has its own view of the world, and most will have been formed by gods that want to do their very best to obscure the actual mechanics of divine power. After all, the optimal power to worshiper ratio is a achieved when you are a cult of one.
This, I think, is the religious model that I will be rolling with. Now I just need to find some time to actually make that starting area.