The high-priest of Zoroastrianism, Kartir Hangirpe, believed that he represented the one true religion. He was an absolutist,
believing that there was good and evil, with nothing in between. Into the later half of the 200s CE, he continued with his persecution of competing religions: the Manichaeans, Christians, Jews and Buddhists. Then, sometime during the reign of Bahram II (276-293), Kartir died, and religious tolerance began to reassert itself.
Bahram II was relatively tolerant. He had been influenced by his grandfather, Shapur I. He had become acquainted with Hellenistic culture — with breadth of mind being the bane of bigotry. Bahram II was offended by the zealotry of the Zoroastrians. As with the other autocracies, freedom to worship depended upon the whim of the ruler, and for awhile persecution was lifted from the Manichaeans, Christians and Jews.
This changed decades later under a new king, to be known as Shapur the Great. In 337, Shapur the Great broke the peace with the Roman empire that had been established for 40 years. Christianity was in power in the eastern half of the Roman empire with whom Shapur was dealing, and to many Persians this made Christianity an enemy religion. Shapur championed the Zoroastrian view and began persecuting not only Christians but also Jews and Manichaeans, seeking their conversion to Zoroastrianism. Entire villages of Christians were slaughtered. Then Shapur restricted the attacks against Christians to priests, monks and nuns.
The survival of Christianity appeared threatened, but Christianity in the Sassanid Empire was saved by the same slowness and inefficiency that had taken place in Rome’s attempted extermination a century or two before. In 379 he died and his son, Shapur II, died. His son and successor, Shapur III, freed Christian prisoners, believing they would be of greater value to him pursuing their crafts and paying taxes.
The Shah from 399 was Yazdegerd the First, and he respected diversity and wanted peace between the religions of his realm. The Persians had generations of experience with diversity, which made acceptance of it easy. Yazdegerd’s mother was Jewish, which may or may not have been significant.
Yazdegerd helped Christians rebuild their churches, destroyed during the persecutions. And he sponsored a council meeting of Christian bishops and other Christian ecclesiastics to mend their internal quarrels. The council of Christians created rules and an organizational structure to unite Christians within the Sassanid empire.
The Zoroastrian priesthood tried to pressure Yazdegerd into renewing repression against the Christians, and Christians helped the Zoroastrian cause. Fervent believers, Christians included, were not always adept at controlling their emotions for the sake of a sound political strategy. Their hostility toward the Zoroastrians led to aggressions. They described Zoroastrianism as the work of the devil. Christians attacked the Zoroastrian priests and destroyed Zoroastrian fire temples. With this, the terrorism of zealots, Yazdegerd’s toleration toward the Christians ceased.
During the reign of Yazdegerd’s son, Bahram V, persecution of Christianity intensified . Many Christians fled westward to the Roman Empire, and Bahram sought their extradition. But the Roman emperor at Constantinople, Theodosius II, refused Bahram’s request. War followed. Constantinople overpowered Persia’s forces in a series of skirmishes, and Bahram made a hundred-year peace with Constantinople, and the peace included his agreeing to grant freedom of worship for Christians in the Sassanid empire in exchange for Constantinople granting freedom of worship for Zoroastrians under its rule. The negativity of war had produced a desire for tolerance — the same that was to happen on a great scale in 17th century Europe.
But not all persecution had ended. The Sassanids still ruled in Armenia, and under a new king, Yazdegerd II (438-457) another attempt to force the Armenians to give up their Christianity. Intolerance was deeply embedded in Zoroastrianism, as it was in Christianity. Zoroastrian missionaries were sent to Armenia in great numbers, and there a systematic persecution of Christians and Jews began.