I think generally it's only capitalized if you're talking about the Jewish/Christian God.
I go with any individual being; the Deity of any monotheistic religion, so in the context of (e.g.) Mithraism I'd talk about God with capitalization.
But also, Thor is an individual, as is Shiva, etc. So I might say, "of course Shiva put the beatdown on a few uppity naagloshii -- he is a God, after all!"
But the Norse pantheon of gods? That I don't capitalize. Unless I suspect they'll be reading what I write, of course. But that's the Archive's schtick, so no worries (she's not a god).