Sorry to beat this horse again. I forgot one point. I'll make it and then put this topic on the back-burner.
Let's put aside the Priest unit for a moment. What of Arcane Barges, or Amurite Firebowmen? Do they too draw their powers directly from their gods? If so, why is there no religion requirement? If not, how do these units manned with 'commoners' learn magic spells immedeately it takes student of the arcane years to learn? And without a mana node to draw on? Or vampires, do they draw power from their gods? (Even Orderly vampires?
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Now Fire magic is very popular with player and AI alike. So most nations that field Fire Barges probably have a Fire node. (The Amurites might even get Fire on their Palace; I didn't look it up.) And there are no diplomatic penalties for Fire nodes. But Death and Entropy nodes do have a penalty, and few civs build such nodes. Yet the Calabim, or any AV civ, can cast massive amounts of Death magic without drawing the ire of Good.
The biggest effect would be on Priest units, agreed. But there are other units in the mix too.
Like I said, I think it would probably be enough if it was possible to program these diplomatic implications based on actual spell casting. But I suspect it is tied to ownership of certain mana nodes due to programmer sanity considerations.
I also doubt there will be much traction to this idea, so I am not going to blister my fingertips typing up advocacy for this point. It just bugs me enough that I have to bring up the topic FWIW. Then the itching goes away.