Them D&D players sure love polytheism. How are we gonna break it to them?

Historically, religion was broadly oriented around racial lines. That is: humans, Sylvans, and Elementals each had their own thing going on. For the latter two, it was generally an animist religion: there are numerous god-like beings dotting the landscape, fitting the expectations of Sylvan and Elemental folk, and thus they venerate them. "Worship" is a bit of a strong word.

Similarly, humans brought their pantheon with them (from...wherever?). Either they never existed, or they were ancient legends obfuscated with time, but they didn't show up when it was time to clash with the much more physically-present Sylvan and Elemental gods. So, in time, they learned to acknowledge these new gods, and the older ones became myth and superstition.

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Ideas:

• Each city has its own god, kind of like a giant threshold spell.
• The Devakin are Highlanders of a sort. There's only so much god-stuff, and they can absorb it from other fallen Deva.
• The gods were slain recently, causing the Deva to come into existence?
  • About 20-25 years ago, there was a great war called the Godswar, in which things got so bad everyone brought out their gods, and most of 'em died. Without their overwhelming power, the brief, decisive war some envisioned became a brutal slog, especially with the power of modern industry behind it.
  • Perhaps this led to the ban on city-fighting, and on cities even being fair game for war. It's one thing for nobles to meet in the field with armies--everyone there is a volunteer or otherwise has skin in the game. But cities in total war just become mass graves, and for what? The honor of this or that house?