@Havenstone
Sure it could be internally consistent, but it would be internally consistent with a lot of propaganda which had a tenuous at best connection to reality. My ability to believe that this is “Our world, but with literal demons” is not strong enough to support the idea that Renaissance propaganda campaigns reflect something about most Protestants (or most Catholics, or most Jews, or most Frenchmen, or most Irishmen, etc.). At some point we get things like “If you want to do a setting involving orcs, why are you calling them ‘Irish’?”, and that point invariably gets more ridiculous the more you make one side unambiguously right in all its propaganda.
As for Mesoamerican priests:
Shoelip mentioned pagans. I specifically picked a fictional pagan who is, to put it mildly, a cruel, malicious dink.
I think him worshiping demons is entirely plausible. Not because he’s a pagan, but because he is the sort of person who would do that.
@Shoelip
If you wanted me to pick whether I thought Montezuma or Cortez was more likely to turn to demons for power, I’d like to think I wouldn’t surprise anyone to say I’d bet on it being Cortez.
But I picked a specific fictional character as an illustration of who I do think makes a plausible demon worshiper - I find “a Mesoamerican priest like him” (as in, this particular individual) to be a lot more plausibly a demon worshiper than “Let’s take Martin Luther, but instead of whatever actually went on in his head, it was demon inspired! Because . . . Catholics are right!”
Borgia as a demon worshiper:
Look at his historical record and his ambitions and tell me that “What wouldn’t he do for power?” never strikes you, or “What, if any, moral scruples actually restrained his use of it?”
To quote from his entry in Barbara Tuchman’s The March of Folly: “So many had been Alexander’s offenses that his contemporaries’ judgments tend to be extreme, but Burchard, his master of Ceremonies, was neither antagonist or apologist. The impression from his toneless diary of Alexander’s Papacy is of continuous violence, murders in churches, bodies in the Tiber, fighting of factions, burnings and lootings, arrests, torture and executions, combined with scandal, frivolities, and continuous ceremony - reception of ambassadors, princes and sovereigns, obsessive attention to garments and jewels, protocol and processions, entertainments and horse races with cardinals winning prizes - with a running record throughout of the costs and finances of the whole.”
Adding “And to strengthen (or acquire) his position, he sold his soul.” doesn’t really come off the same way that “The Ninety-Five Theses was the product of a mind influenced by demons” does. Alexander VI was, at best, not taking his religious duties very seriously.
@Pilgrim
You could have it grant eternal life to “the worthy”, with a definition of “worthy” more like how Exalted in the game of the same name are people destined for greatness - which may or may not have any relationship with “goodness”.
That would allow the people you want to benefit from it to benefit (hopefully all PCs would count - who wants to play a loser in a game like this?) and the people you don’t would somehow not measure up.
Could go for something else, depending on how you want to present it, but while I’m giving opinions (and realizing that phrasing it as “opinionated” makes it sound more dogmatic than I intend - its not my game and I don’t want to make it my game, nor dictate other people’s beliefs) that’s my opinion.