I always thought the Amarr is what happens when you smush all the monotheistic religions into one, let it sit for a couple thousand years, and then unleash it into space. Not just Zoroastrians and Catholics(which are both essentially 1 Deity vs 1 Demon religions), but also the entire, convoluted spectrum of the Judeo-Christian-Islamic faith, and all the smaller, less-well-known single-God religions.
Brazenly against the concept of the existence of other gods (Polytheists from around the world have a strange tendency to reluctantly accept the idea of the existence of other gods than simply those in their personal pantheon, but strongly believe their selection of Gods to be the strongest and most righteous), often to the point of militarism, they have a tendency to believe in the idea of divine selection over inherent spiritual power (Saints, Prophets, Chosen Ones, etc vs Ancestor Spirits, Wizard/Witchdoctors, Heroes, etc), and, of course, organize themselves into a deeply hierarchical religious system.
Now give them pseudo-Persian aesthetics and a lot of phallic architecture, and voilà! Amarr!