The Achaemenian kings could rule their vast empire only because they respected and officially accepted the religions of the different city states and countries as their own and satisfied the clergy in the conquered countries. In places where this was neglected at certain times- as for instance Egypt under Cambyses - revolts occurred. The Aryan tribes had accepted Zoroastrianism by the time of Cyrus but without imposing it on other nationalities. The palaces at Persepolis served the Zoroastrian spring festival and New Year’s rites and bear numerous Zoroastrian symbols.
Zoroastrian religious thought was an expression of Iranian tradition. It proclaimed the holiness of fire, water, air and earth, which could not be defiled by anything unclean. The world and the heavens were in a fight of good (Hormuzd) against evil (Ahriman), of the truth against the lie, of light against darkness. It was hoped that the good side would win, but this victory was not a foregone conclusion, and the active participation of the committed individual was required. The Parthian kings accepted Zoroastrianism, which continued to be practiced in Iran. At the same time several new religions prospered, foremost Christianity, which started to take a hold in Egypt and in the Greek portion of the Roman Empire, and flourished in the Parthian Empire among the Assyrians and among the Armenians. Christianity was tolerated in Parthia in the same way all other religions were, receiving no special preference or prejudice.
One of the most important places where Zoroastrianism was practiced was the city of Istakhr, outside Persepolis. One of the Zoroastrian high priests, Sassan, lived there (A.D. 180). He was succeeded by Papak who established himself as King of Fars under the Parthian King of Kings. This occurred during a period of internal dissension amongst the Parthians and continuous fights with the Romans on the western border. Papak’s son Ardeshir, announced his independence from Artabanus V, symbolically, by building a palace of royal pretensions at Firuzabad during one of the more successful wars of the Parthians with Rome. Artabanus turned cast to quell the rebellion only to lose his life in A.D. 224 in a battle with the forces of Ardeshir.