Persians at the time in question were monotheistic and followed the practices of what is now termed Zoroastrianism, but which would have been called Mazdayasna at the time.
Basically, they held that there was one god, Ahura Mazda, from whom sprang all things good. In contrast, there was an uncreated evil one, Angra Manyu, from whom all evil sprang. The evil forces were constantly trying to convince mankind to undermine the good of Ahura Mazda, and given that both forces are uncreated, the only thing that would ensure the eventual triumph of righteousness 
was the faithfulness of the people. There were lesser deities (think angels and demons) that held dominion over certain aspects of creation. Most of their beliefs were based on the prophetic text of Zoroaster (another name for whom is Jartosht, which is how I refer to him in Jewel of Persia).
That’s a nut-shell version. Now, though the tombs of Darius and Xerxes both paid homage to Ahura Mazda as the only god, it’s worth noting that Artaxerxes’ tomb (he succeeded Xerxes) mentions three gods. This leads me to believe that Mazdayasna was losing ground with the people in this age.
Though Herodotus tends to have the Persians making sacrifices to “the gods” and doing other things typically Greek, the speech of the Persians in his work still reflects the appropriate belief system—they’re constantly referring to “the god.” It was “the god” who visited Xerxes in a dream, “the god” they were always trying to please.
It’s also worth noting that “the god” spoken of in Herodotus bares little resemblance to Ahura Mazda in the works of Zoroaster. I have Xerxes acknowledging this at one point, which is what leads him to abandon his faith in the god. (This is not recorded—it’s my way of bridging the gap between Herodotus, Persian history, and the book of Esther. Pure fiction, though reasonable and possible.)
For all their monotheism, the Persian kings extended religious freedom to their subjects. In many ways, theirs was a culture with ideals shared by us moderns.
The idea that Xerxes and other Persian rulers were god-kings has its roots in earlier times of the empire and shaped their law–this is why Xerxes could not undo any law he made. A god can’t go back on his word, right? But by the time Xerxes ruled, Mazdayasna had obliterated the idea of a god sitting on the throne, and this was tradition only, not actual belief. Though Xerxes was still so loved and revered that it could be argued people worshiped him.
(Picture of a Faravahar, a symbol of the religion thought to depict one of the guardian spirits.)
Return to Companion Guide