Zoroastrianism had one God named Ahura-Mazda who is benevolent & created world based on cosmic and social order with an emphasis on truth. Persian kings viewed it as duty to be responsible for people. Zoroastrian idea of global sovereignty of God & responsibility in maintaining cosmic & society's moral order led to concept of global empire as instrument or expression of the religious order of the world that Persian king was charged with maintaining. As Persia's power grew after 500 CE and conquered diverse people the empire's mission was expressed as bringing justice as defined within Iranian society's religion and culture which meant imposing one rule and one language. To this point Darius created 3 satrapies in Egypt in an administrative change based on language - this is similar to Achaemenids division on cultural not geographic lines for governance and taxation.