Despite the overall policy of religious tolerance, there was still an official Persian state religion: Zoroastrianism. Zoroastrianism, named after its prophet Zoroaster, taught that the world was being fought over by two great powers: a god of goodness, honesty, and benevolence known as Ahura Mazda (meaning “Lord Wisdom”) and an evil spirit, Ahriman. Every time a person did something righteous, honest, or brave, Ahura Mazda won a victory over Ahriman, while every time someone did something cruel, dishonest, or dishonorable Ahriman pushed back against Ahura Mazda. Thus, humans had a major role to play in bringing about the final victory of Ahura Mazda through their actions.
Zoroaster himself lived in around 750 BCE, before the rise of the empire, and was responsible for codifying the beliefs of the religion named after him. Zoroaster claimed that Ahura Mazda was the primary god and would ultimately triumph in the battle against evil, but explained the existence of evil in the world as a result of the struggle against Ahriman. Thus, Ahura Mazda was not “all-powerful” in quite the same way as the Jewish (and later Christian and Muslim) God was believed to be. Human actions mattered in this scheme because everyone played a role, however minor, in helping to bring about order and righteousness or impeded progress by indulging in wickedness. Zoroastrianism also told a specific story about the afterlife: when the power of good finally triumphs definitively over evil, those who lived righteously would live forever in the glorious presence of Ahura Mazda, while those who were evil would suffer forever in a black pit.
There are obvious parallels here between Zoroastrianism and Jewish and Christian beliefs. Indeed, there is a direct link between the Zoroastrian Ahriman and the Jewish and Christian figure of Satan, who was simply a dark spirit in the early books of the Torah but later became a distinct presence, the “nemesis” of God Himself and a threat to the order of the world, if not to God. Likewise, the Christian idea of the final judgment is clearly indebted to the Zoroastrian one: a great day of reckoning.
In turn, Zoroastrianism provided a spiritual justification for the expansion of the Persian Empire. Because the great kings believed that they were the earthly representatives of Ahura Mazda, they claimed that the expansion of the empire would bring the final triumph of good over evil sooner. There was a parallel here to the beliefs of the ancient Egyptians, who had also (during their expansionist phase during the New Kingdom) claimed to be bringing order to a chaotic world at the end of a sword. The kings sponsored Zoroastrian temples and expanded the faith at least in part because the faith supported them: the magi, or priests, preached in favor of the continued power and expansion of the empire.
One noteworthy aspect of Zoroastrianism is that, in contrast to other ancient religions (including Judaism, and later, Christianity), Zoroastrianism appears to have banned slavery on spiritual grounds. This is important to bear in mind in the context of discussing the Persian War, below. The Greeks thought of the war as the defense of their glorious traditions, including the political participation of citizens in the state, but it was the Greeks who controlled a society that was heavily dependent on slavery, whereas slavery was at least less prevalent in Persia than in Greece (despite the religious ban, slavery was clearly still present in the Persian Empire to some degree).