Ardashir I overthrew the last Parthian king around 224 and founded a dynasty that made Zoroastrianism the religion of the Iranian state in a way no previous Persian empire had. The relation was expressed as a proverb the Sasanians themselves used: kingship and religion are brothers, and neither can stand without the other. Fire temples were founded and endowed across the empire, the greatest of them Adur Gushnasp in Media, associated with the warrior estate, and Adur Farnbag and Adur Burzen-Mihr with the priests and the farmers, and kings made pilgrimage to them on foot.
A hierarchy of mowbeds administered the religion, and its most visible figure is the priest Kirdir, who served four kings in the third century and had his career carved into the rock at Naqsh-e Rostam and elsewhere. His inscriptions record the founding of fires, the promotion of priestly discipline, an account of a visionary journey to the next world, and a list of the communities he says he struck at, among them Jews, Christians, Manichaeans, Buddhists and Brahmins.
Mary Boyce's work established how much of the later Zoroastrian tradition was fixed in this period: the collation of the Avesta, the elaboration of the calendar and the purity law, and the temple cult of fire that the Parsi communities of India still maintain.