The Seven are fairly similar to Christianity. They’ve got a doctrine and a fairly similar structure, and even wandering priests who go from village to village, performing marriages and preaching. There was some talk in some of the non-book material that, as they acknowledged half the faces of God were female that they their Septas had more power than RL nuns did, but if so, that hasn’t shown up much.
The Old Gods are seem vaugly animist. They have no doctrine, just a few taboos. No kinslaying, no oathbreaking, no violation of guest-right, past that, you’re pretty much on your own.
The Drowned God = Cthulhu is funny, but really, aside from appreciating the occasional human sacrifice, he doesn’t seem any worse than most of the other gods. He’d fit right in with the [del]Dorathki[/del] [del]Doraathkai[/del] [del]Dorathhakki[/del] Horselords. Who seem to have a very vague polytheism (with the Great Stallion on top) mixed with some ancestor worship. They get a kick out of beating up other gods and taking their stuff.
The Tygarians were atheists, or so the stories go. Dany doesn’t seem particularly religious, though has a bit of curiosity about the Seven.
The Red God is a bit like Zoroastrianism, or a dualist christianity where Satan is God’s equal. They have POWERS! Which is impressive, and sets them apart . . .
But the thing is, a lot of other groups have powers, too, and not all are religious. The Wargs of the North, for instance, and some sorcerers in the east. The Tygarians and other Valarians, of course. Miri ‘worshiped’ the Lamb God, but she learned magic from a Shadowbinder. (I’m not sure what a shadowbinder is, but it doesn’t sound like the fun kind of ‘binder’)
Southron Westeros has the Alchemists/Pyromancers, who used to have more powers than they do now. They were still better at ‘fire magic’ than Thoros when the books started. The Old Gods used to have their own set of priests and sorcerers. The Maesters actually offer ‘magic’ as a major. Which doesn’t work anymore, but used to.
So the Red Priests powers, impressive as they are, are not really proof of anything.
The Maesters are a political and educational organization. They’re not inherently a religious order, but they’re tied strongly to the Seven worshiping parts of Westeros. They don’t seem to encourage devotion.
And there are a LOT of others. Most every group seems to have at least one patron god, and as ‘National Georgraphic’ stories get more common, you’ll see more and more.
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Poor Binders. Not as good as a regular Warlock, OR as a Trapper Keeper.