I love movies and books about the mystical nature of religion. Constantine, Stigmata, The Seventh Seal, The Prophecy…I just cannot resist them. The other day I watched a movie trailer for Legion, in which God sends his angels to exterminate the world’s population. The archangel Michael (played by the wonderful Paul Bettany) is the only one standing between mankind and the apocalypse. I whispered, “Ooh, I want to see that.” My teenager, sitting next to me, frowned. “If God’s tired of humanity,” he mused, “why go to all the trouble of sending the battling angels? Why doesn’t He just snap His fingers and make the people disappear?”
He was right. If an omnipotent being is tired of his creation, why would he bother with the trouble of watching them run and scream and die? Isn’t that a lot of effort for no real return? This is the problem of using an omnipotent being and trying to limit it.
Religion is an important part of worldbuilding, and you should probably give it at least as much thought as your magic systems. Every culture in our real life has its own beliefs and rituals, some rudimentary, some highly sophisticated. It has great influence over some societies, while others treat it as just another thing to do. In your fantasy culture, it’s up to you, the author, to decide how much power the religious community and the god they worship wield, and then to display that power properly in the narrative. Even if your characters aren’t particularly religious, it’s a good idea to weave those aspects into the story, for depth and richness. My pirates, for example, occasionally mention a god or two, but there’s no real devotion shown to them. I, the author, know exactly who those gods are, what their spheres of influence include and what they can be expected to do when their worshippers ask. It didn’t come into play in the story, so the reader didn’t see it. But I definitely know.
So how powerful do you want your gods to be? This depends entirely on how the story needs to play out. There’s nothing wrong with using a god as a character, or for human characters to call upon and employ a god’s power. But think about that whole omnipotence problem I mentioned earlier. If your god is so powerful that he can do anything and everything, it might not make a lot of sense for him to bother with sending others to do his bidding. Takes more energy that way, you know. If you do come up with a reason for an omnipotent being to take the long way around to reach his ultimate goal, be sure you make it clear in the narrative.
And making the god’s avatar look like Paul Bettany doesn’t hurt.