How did Zarathushtra’s teachings reform older Persian polytheistic beliefs?

Zarathushtra, a priest of the ancient Iranian religion, sought to reform the existing polytheistic traditions rather than overthrow them entirely. His reforms were centered on the following key shifts:1

• Elevation of Ahura Mazdā: Zarathushtra portrayed Ahura Mazdā as the supreme creator and the only deity worthy of worship. He organized the divine realm by surrounding this "Wise Lord" with an escort of entities—the amesha spentas—which represented the powers of other gods in a unified kingdom of justice.12
• Ethical Reorientation: He redefined the relationship between the divine and humanity by establishing that both must observe the same ethical principles. Followers were encouraged to embody qualities like truth, justice, and righteous thinking, effectively binding the world of the deity to the world of his followers, the ashavan.1
• Rejection of the Daevas: Zarathushtra’s system was explicitly hostile to the cult of the daevas, which were prominent old Indo-Iranian gods. He relegated these beings to the realm of evil spirits who chose to oppose the truth.12