What have I always believed?
That on the whole, and by and large, if a man lived properly, not according to what any priests said, but according to what seemed decent and honest inside, then it would, at the end, more or less, turn out all right.
-from Small Gods (Terry Pratchett)
Religion or Cult?
First off, eh, you’ve got to decide what purpose religion serves in your narrative or to your characters. Very important question, and an important part of that is to decide whether they’re working with a religion or with a cult. Unlike some definitions, a ‘cult’ is not a ‘religion with few members’, and here’s why: Religions are all the same.
What? Yeah. They’re all based on a general morality that boils down to 1. Don’t Murder and 2. Don’t Steal both of which derives from some form of “Do to Others As You’d Want Done To You”. The ole’ Ethic of Reciprocity. Then each religion dresses it up with fancy outfits or specific restrictions. Your basic cult, on the other hand, leaves that general morality out, and fills the void with crazy.
You can see from the definition that at some points in human history, some of the great religions have most certainly been cults for some of their worshippers. The important difference, and the reason they’re still around, is, of course, that eventually enough worshippers who weren’t crazy were able to call out that cult-behavior as perversions of a great religion.
Bringing it into writing: Cults make convenient antagonists, but not well-rounded characters. Crazy thinks in open figures (sometimes literally), anything can get in; Religion is a polygon of virtue.
“I’ll tell you one thing I’ve learned here already.”
“When I’m in charge, if someone knocks, they will be able to come in. Making someone who is seeking comfort stand out in the cold is a crock of rancid yak butter.”
-from Lamb (Christopher Moore)
Religion Evolves & Creed Accretes
Pardon my Culinary, but the big thing that sets forkin’ religion apart from mere cultlery is Flexibility, and Time in which to be flexible. Defining a ‘Grandfather’ as 40 years, a 100-grandfather religion will not look the same now as it did at the first grandfather, and you can safely bet that even before the 50-grandfather mark, new religions were starting to spin off. Sure, it may seem that religion is sometimes a grandfather behind the times, but that’s sometimes because the grandfathers are still alive.
That is to say, every generation eventually gets the religion it wants, or it gets a cult. Either the generation adapts the religion to the times, or they go crazy, at which point the next generation picks up the pieces or the religion dies. It’s hard to sustain crazy across generations. That’s one of those sentences that’s both optimistic and depressing at the same time.
Philosophy or Religion?
Character-wise, there’s a big difference between Religion and Philosophy, and it comes down again to flexibility: Philosophy is a buffet, Religion is a set course. A character who has a philosophy will pick and choose whatever works to build a framework around whatever he’s doing. A religious character will stick to the framework he believes in already. Sometimes this will lead the religious to getting Canadian bacon instead of real food, and sometimes this will lead the philosophical to fill up on bread, but they’ll both get fed eventually. Morality all the same going down, it’s just a matter of quality of ingredients, preparation, and, of course, presentation.
A strange feature was that the ordinary ants were not excited by the songs, nor interested by the lectures. They accepted them as matters of course. They were rituals to them, like the Mammy songs or the conversations about their Beloved Leader. They did not look at these things as good or bad, exciting, rational or terrible. They did not look at them at all, but accepted them as Done.
-from The Once and Future King (T.H. White)
Culture is Not Religion
It’s important to remember that in any real question of morality, culture will trump religion. Religion might color the argument a bit, but it all comes down to what people around you do and what that means for you. Every society has had atheists, whether they admit it or whether they admit it, and yet the atheists have been part of the culture, which is the important bit.
In short, it is not a cynical statement to say that The Crusades were only a Religious War for those who didn’t turn a profit.
Hierarchy, Decentralized, Independent
Thus, culture will determine the shape that religion takes, and not the other way round, and religion will evolve with the culture. The less said on that, the better, and a quick look at the major sects of the various religions will be enough. Just cross-reference it with a cultural time-line.
Christianity - Catholic, Protestant, Mormon
Islam - Sunni, Shi’ite, Sufi
Buddhism - Theravada, Mahayanna, Tibetan
Judaism - Orthodox, Reform
Bringing it back to writing, there are very different character types that can spring from the different types of religions (which are themselves mere reflections of culture). There’s only one real choice for writing, and that’s what’s most interesting. A hierarchy will spawn those who think of hierarchy, talk of duty and the inevitable corruption of any organization, but also stability and a love for stability. It also helps that presumably everybody’s heard of it. An independent religion will avoid much of the corruption by losing the stability and nobody’s heard of it, but it’s flexible. If you’re making it up anyways, a decentralized religion is the best way to go, because you get all that fun corruption while retaining enough flexibility for your characters to play with.
God As A Character
In the sense of a omniscient, omnipotent entity...just don’t do it. If you don’t understand the motivations of all your characters, it’ll just seem weird. The best you can do is dress it up as an ineffable wonder. Anyways, as a writer, you’re God, so don’t make up competition for yourself. This goes hand-in-hand with the decentralized religion approach above, on account of everybody likes a good pantheon of human-like gods. Easier to keep track of, if you really must.
In the fine traditions of writing advice, I've already broken the God As a Character Rule. To see how it went, read: "New Sodom Takes Her Chances"