Parthian Persia was tolerant of religions and home to many faiths. Hindus, Buddhists, Greeks, Jews, Christians, and pagans mixed there. Under the Parthians Zoroastrianism itself had great variety. In a huge country with little communication, many ways of practicing the religion had developed. Some people worshipped Ahura Mazda in fire temples, others turned back to Mithra, an ancient god who became one of the archangels of Zoroastrian belief. Some temples had statues of Anahita, a female deity. The Parthian kings themselves followed a form of the religion that had grown up in the east, but they did not try to impose their beliefs on the people. Instead they were tolerant of all forms of worship.
In the first century c.e. the Parthian rulers began collecting the Avestan materials that had been scattered during Alexander’s invasion and writing them down. Some materials, like the Gathas, had survived more or less intact. Others were in bits and pieces, fragments of different rituals and beliefs. The Parthians set about gathering everything they could find. Centuries of warfare with the Romans weakened the Parthians, who were overrun by Roman armies for more than 80 years before they finally gave way in 198 c.e. The Parthian royal line continued, but with little power. On the whole, however, Parthian rule had helped to strengthen Zoroastrianism.
As the Parthians declined another Persian empire was on the rise. This one was ruled by Ardashir, the grandson of the Persian noble Sassan. Ardashir also claimed to be descended from the Achaemenids. He defeated the last of the Parthian kings in 224 c.e. and consolidated his power over what had been their territory. Once he had brought all the Iranian kings and subkings under his control, Ardashir took on Rome. Shapur I, Ardashir’s son, continued his work, defeating the Roman emperor Valerian in 259. In matters of religion in general and Zoroastrianism in particular, the Sassanians thought the Parthians had been too liberal. To discourage the bubbling mix of religions on their soil, they made Zoroastrianism the official state religion. Then they set about standardizing it. Under the Parthians followers of different priestly traditions had developed varying forms of ritual and belief. The Sassanians decided that there should be only one. They created a national priesthood to examine the materials the Parthians had collected and to do away with any ritual forms they felt were not “pure.” Through this priestly editing a standardized form of Zoroastrianism appeared. Some variety and vigor were necessarily lost, and the religion became more concerned with ritual than it had previously been, but now it was clear what was and was not Zoroastrian.