This post discusses the creation and use of fictional religions for video games/fictional worlds. Please do not view it as an attack on your real-world religions or religious beliefs.
I've played a lot of fantasy games, read a lot of fantasy stories. They tend to have a lot of gods in them.
Normally, these in-world religions fall into two camps. 1) Religions that are funhouse mirrors of real-world religions (AKA the "crystal dragon space jesus"). 2) Religions whose pantheons exist to cover the facets of the game world that the players will care about. IE, simplifying your pantheon into "goddess of healing", "god of thunder", "goddess of trade", "god of evil", etc.
I don't really have a problem with the first approach, if you're doing it on purpose. However, the second approach is really pretty boring. So let's talk about how you might create some fictional religions that have some real heft.
Religions are not gods. This is a critical point to make. You can have gods without religions, and religions without gods. You can also have religions which have betrayed their god, or their god was killed, or whatever.
Religions serve a purpose to a group of individuals (not necessarily people). A god is just a super-powerful entity. The only time the two are very tightly linked are if you are assuming prayer-powered gods, a concept which was briefly popular in the late 90s and early 2000s. Nothing wrong with that idea, has some fun meat to it, but let's not limit ourselves to it.
So lets deal with religions and gods separately.
Here's my thoughts on gods!
Most of the time, our stories do not need gods that are extensions of religions, or religions that are extensions of gods. Instead, we need to think about A) what a god is and B) what purpose the religion serves.
In many fantasy settings, the gods are basically just big people. Big, immortal people. If that's the case, that's fine. However, if that is the case then it doesn't make a whole lot of sense for a god to have a single, specific domain.
Sure, Thor was the god of thunder. And he liked being the god of thunder. But he also had a hand in storms, oak trees, strength, protection of mankind, hallowing, healing, fertility, and drunken binges.
In the real-world religion, these were added to Thor as he merged with various predecessors and cult gods, in the same way that Tyr was demoted from highest god to "that guy that guards the bridge". Unless your fantasy religion's god pops down to say "hey, actually, I don't do that kind of thing you just prayed for, try the temple down the road", people will keep attaching new domains to their favorite god. Eventually, a favorite god can be promoted to "the high god" or "the only god".
In a fantasy religion, Thor could be an actual person running around doing things, and it's easy to see that his domain might end up much the same. It's the sort of eclectic mix you might get by looking at a real person's interests. So the two (worshipping a human-like god and letting a religion grow over time) can have very similar results if you like.
On the other hand, there's a lot to be said for gods that aren't human-like. Or maybe some gods that are and some gods that aren't.
Some of the most compelling gods in games have been gods of ancient religions that have a very alien tint to them. Not all the details can be teased out. Was Xikobaz the goddess of darkness? Well, the translation isn't so simple, and parts of the tablets have been broken, but that thing coming out of the well is on fire, so maybe not?
When you're thinking of a god in a fantasy game, you need to decide what sort of manifestations the players/audience is likely to find.
If you've got human-like gods, you may actually run into them out in the world, maybe even fight them. That can be fun.
But as you start stepping away from human-like, you start getting gods whose powers are more subtle. Maybe you'll meet Cmirkl, ancient sleeping god of shell and bone. But even before you meet him, you'll see the creatures his power creates, the landscapes his existence twists, the temples built to him by people long forgotten by time... or perhaps by people who failed to exist at all, in the end.
Generally, I rank gods on a "queasiness scale".
1) Just a big, superpowered humanoid. Or animal, whatever. Probably has an eclectic mix of powers. Example: Thor, Superman.
2) Seems like a big, superpowered humanoid, but has some crazy powers hidden underneath, such as time travel or infinite polymorphing or dream walking. Example: Loki, Odin.
3) May appear in a humanoid form, but only to interact with humans. Powers leak and bleed enough that being in their presence is probably not wise. Still, recognizably a god. Example: Crystal Space Dragon Jesus, Gaia, Dream.
4) Does not exist primarily in our universe, and doesn't really understand much about humans. Often called "sleeping", because "ignoring us" really doesn't sound so good. Bleeds power like a sieve, can be seen as either a god or a force of nature, depending on how you squint. Example: Cthulhu (sometimes).
5) Largely incomprehensible. Maybe dead, or doesn't exist yet? Anyway, treat it like a bizarre force of nature, not a god. Example: Cthulhu (sometimes), King in Yellow.
Now, what purpose does the religion serve?
1) Cargo cult bullshit. Try to deal with the chaos by worshipping. Has no real effect.
2) Contains the god. Or wards them off. Or sates them. The religion keeps the god from going ballistic on you.
3) Curries favor. The god gently exerts pressure in favor of their worshippers. This is a small but noticeable actual advantage. Otherwise it's just cargo cult stuff.
4) Explicit favors. The god grants magic, miracles, personal appearances, sends angels, whatever. The power of this religion is absolutely undeniable.
5) Respect. The religion doesn't offer up much in the way of powers, but exists out of respect for the deity. Functionally, this usually means the religion is a storehouse of best practices for a given domain, or for emergency responses when something takes a supernatural nose-dive.
Religions are fundamentally about the people doing the worshipping. You can say that a religion is a set of beliefs, rituals, and general practices. However, those don't exist separately from the practitioners. As time passes, the religion evolves in tandem with the civilization it is part of, and some of the older beliefs, rituals, and practices are downplayed or forgotten while new ones come around.
Since this is a fantasy setting, there may be some magical enforcement. For example, it could be that you worship this idol because if you don't, it will kill you. Really - if you miss a day, it will come awake at night, hunt you down, and stab you with a mystic spork.
However, in the absence of such pressures (and even with such pressures), religions still exist in a cultural setting, and serve a cultural purpose. To that end, we can split our religions into a few basic types, all of which can be monotheistic, polytheistic, deistic, etc.
1) Stable state religions. These are religions where, in theory, everyone is part of the same religion. These are typically very old, and are often the source of stability in societies more than 100 (fictional) generations old. These are typically extremely stagnant. The top is usually rife with corruption unless you're doing an idyllic view of things, but at the level of an individual town's church it can be as oppressive or friendly as your story demands. Keep in mind that really oppressive religions tend to get in trouble and become...
2) Unstable state religions. This is where there is a state religion, but it is losing its grip. This leads to much scrabbling in the form of witch-hunts, oppression, and other nastiness. This is often caused by the rise of other religions (often because of mass immigration), but can also be caused by severe trouble, such as a sudden spike in the number of dragons roaming the land. Either way, this religion is burning all its goodwill very fast, and it is unlikely it will ever recover. Eventually it may become...
3) Fractured religions. This is where the religions have the same core beliefs and share most or all of the same holy books/relic/magic. However, they have some rift between them which causes some of the population to belong to one, and some to belong to the other. How bitter this feud is depends on the age of the split: the older, the milder, assuming the religions continued to occupy the same lands the whole time. The split usually happens for tribal reasons, not liturgical ones - that is, you side with your kin against their kin. The differences are usually a list of very minor issues, plus one side tending to be more strict than the other.
3) Minority religions. When a small but significant number of the population belongs to a given religion. While the majority may not be completely happy about it, a minority religion is not an outlaw religion, and they are more or less left to worship in peace, aside from as much racism as you care to plug into your setting. Keep in mind that what is a minority religion here is often a more pervasive religion in someone's homeland, but the main religion and the branch are often under very different pressures, and their beliefs, rituals, and practices may change in different directions in only a year or two.
4) Outlaw religions. Whuh-oh, don't tell anyone you worship Xegelbaz, or you'll find yourself in the gaol for sure. Maybe the witch-hunt is ongoing. Maybe the practitioners have been rounded up into "work camps". Maybe it's just a standing order to report practitioners, but nobody really expects to find one. In the real world, this often happens due to a conquering religion, but in the fantasy world it may be because the magic resulting from this worship is sinister.
5) Conquering religions. This is a religion which is or was a state religion, but has decided that the world needs to be conquered. Conquering religions will use overwhelming force against other religions and the nations that believe them - the tools of the sword, the economy, and the proselytizer. The way this works is that the old religion is outlawed, while a branch of the conquering religion is created which incorporates some of the conquered religion's practices (a spoonful of sugar makes the religion go down). Please note that conquering religions are often rapidly changing themselves, as well: this is not a stagnant religion.
6) Secret religions. How many worshippers are there? Impossible to know. Often a conquered religion, or an inherited-by-bloodline religion. Any way you cut it, nobody admits it, they all worship in secret. If the fantasy world gives out magic for your religious worship, this can be a very powerful secret!
7) Rebellious religions. A minority religion or cult which decides to enter mainstream by fomenting rebellion, or attaching itself to rebellion which is already fomenting. The "us vs them" logic of a rebellion makes it very, very easy to recruit people and turn them into zealots for your church. If your rebellion succeeds, maybe you'll be a state religion in the new nation...
8) Bored-to-action religions. This arises somewhat rarely, but I thought I'd mention it. When society has plenty of people who are interested in changing (themselves or the world), branches of existing religions may pop up organized around that philosophy. Of course, just as likely are secret societies, NGOs, etc.
9) Cults. Everyone loves a good cult! Please keep in mind that a cult has a pretty specific definition. Rather than use it loosely, I'm using it specifically: this is a religion, regardless of size, which uses at least half of these techniques:
A) Assigns a new name or identity when you join/rank up.
B) Isolates practitioners from nonpractitioners.
C) Promises access to secrets if you can rank up enough.
D) Promises experiences not permitted by cultural norms/laws.
E) Has a single, obsessive leader.
F) Follows one core tenet, insists that all problems can be solved by it.
G) Uses physical or emotional stress to make people vulnerable to suggestion.
H) Subverts the hold of the government over its practitioners via secrecy or bribery.
10) Dead religions. Dead religions are a really interesting topic, and also include any religion which is mostly-dead (<20 practitioners). Because this is fantasy, dead religions may still have a lot of power. For example, stumbling into a temple to the ancient goddess of dreams and hallucinations may not be a very comfortable experience...
If you read this far, you're really, really patient. What do you think?