Religions. Sigh. Can’t live with them, can’t live without them. They have been such an intergral part of Human existance, that ignoring them is something you do at your own peril. A lack of religion (even hidden away in the background) can completely destroy the verisimiltude that a world designer desperately works so hard to achieve. So here is brief outline of various kinds of theisms.
This is a religion where there are many deities, of varying power, and usually many lesser spirits as well. Each seperate “cult” (group of devotees) may decide that its diety is the “best one”, but all accept the reality of the others as genuine divine powers. So while your primary worship may be to your city’s patron deity (or your craft’s deity), you will often turn to worshiping specialist dieties when appropriate, such as the Goddess of Love when thoughts turn to woo-ing.
A given cultural group with worship a specific pantheon - an interrelated group of deities. These deities will fit their cultural needs. Now societies will defend their pantheon as the best and scorn other dieties. Polytheists generally will not ignore these other Gods. After all there are many deities, what is the problem with more. Now as cultures change, new deities will be added. Dionysis was added to the Greek Pantheon and Isis being adopted from the Egyptians by the Roman are prime examples of this.
Now please remember that 99.9% of all the people who have ever lived were polytheistic in worship.
This is the belief that there is only ONE deity. Every spirit thing not of the one deity claiming to be a deity (or a spirit of any power) are demons or fell spirits in disguise. Pure monotheistic religions are rare. Until modern times, they seem not to have existed at all. Most religions that are catagorized as monotheistic have an attendent pantheon of lesser angels, saints, divine relatives, and demonic opposition. Once you extra elements, then you don’t have a real Monotheistic religion.
Now some people have issues with trinities, or the One Diety actually having multiple aspects, being called monotheisms. I personally can’t see telling a divine being of immense power that it can’t have various aspects and still be one deity. So we move on.
This is a belief systems similar to monotheism. In this category, there is ONE important diety, with a bunch of lesser ones who do the One Deities will. These lesser dieties are the angels, saints, divine relatives, and so on. Most so called Monotheisms today, fall into this cateogy.
A related form of Henotheism, where the One Divine Power is divided amongst the two aspects, usually of opposing genders or allignments. The most commonly known Duotheistic religions is Wicca, where the Goddess (Mother Nature/ Gaia) and the God (Horned One) are the primary force. The other, lesser known one is Zoastorism. This was once considered the first monotheistic religion, except its primal force was divided between the One Good Diety and the One Evil Diety, which were both parts of the same coin.
There are tritheistic and quadtheistic possible religions, just none spring to mind.
This is often a modifier for a type of religion. Dualism is the doctrine that all the spiritual forces in the universe are in two camps, Good guys vs Bad Guys and they are constantly at war. Most monotheistic religions were actually dualistic henotheisms or polytheisms.
Spirits, Spirits, Everywhere
Just about every religion admists that there are lots of different spirits around. Each religious type has its own way to deal with these spirits. Monotheisms want only their one Good God to be worshiped, with other spirits being respected or reviled - but never worshiped. Polytheistic relgiongs don’t care about the spirits, as people should worship any spirits they feel the need to worship (since sometime the spirit chooses them). Most Henotheistic and duothesitic religions say that all spirits are part of the primal one, so the worship of any proper spirit is acceptable. If Dualism is in the mix, any worship and respect is reserved for “Good” deities and spirits.
Notes on the interaction of competing faiths
In most cases any faith will compete with any other faith in the same area. There are exceptions like Buddhism and Shinto in Japan, where each one fills an aspect the other is lacking; but generally religions tend to clash in some way and form. It becomes a competition, because as many historians have noted, the religion that controls a group of people also (usually) controls (or significantly influences) the economic, social, political, and military of said group. So the presence of a new way of worship can lead to a struggle over which worship will define the people.
Monotheists have an advantage over most other religious organizations as they are focused to one diety, one cause. The more deities involved, the less focused the grouping can be… because each lesser deity has its own sphere of influence.. and that influence may not be appropriate. Thus polytheists never think about turning to religious genocide.