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Zoroastrianism

Under Parthian rule, Zoroastrianism had fragmented into regional variations which also saw the rise of local cult-deities, some from Iranian religious tradition but others drawn from Greek tradition too. Greek paganism and religious ideas had spread and mixed with Zoroastrianism when Alexander the Great had conquered the Persian Empire from Darius III —a process of Greco-Persian religious and cultural synthesisation which had continued into the Parthian era. However, under the Sassanids, an orthodox Zoroastrianism was revived and the religion would undergo numerous and important developments.

Sassanid Zoroastrianism developed clear distinctions from the practices laid out in the Avesta, the holy books of Zoroastrianism. Sassanid religious policies contributed to the flourishing of numerous religious reform movements, most importantly those founded by the religious leaders Mani and Mazdak.

The relationship between the Sassanid kings and the religions practiced in their empire became complex and varied. For instance, while Shapur I tolerated and encouraged a variety of religions and seems to have been a Zurvanite himself, religious minorities at times were suppressed under later kings, such as Bahram II. Shapur II, on the other hand, tolerated religious groups except Christians, whom he only persecuted in the wake of Constantine's conversion.