[Overcast]

I figure if you stick around the Evil Return central power structure being biblical you can always pitch the near-humans with the Nephilim. If you'd rather be vauge with their background and make the demons the sideways element the idea that Demons are a dimension off seem to make Exorcism work just about anywhere. To exist a demon has to exert its willpower onto the mundane world and exorcism is just the mental contest between the two parties.

I'm sure you'll find something, just give it some thought.