Zoroastrianism, is the religion founded by Zoroaster, or Zarathustra, in Persia toward the end of the seventh century BCE. Today, obscurity still surrounds many details of the personality of Zoroaster, as well as the time and place of his preaching. Scholars have dated his birth in the latter half of the seventh century BCE and his death seventy-seven years later. When he was thirty years old, he began his ministry, but for ten years he had only one disciple, his cousin. His progress began when he converted his doctrine to a local ruler, Vashtaspa. Under the royal auspices, disciples rapidly increased, but holy wars developed between believers and unbelievers. In one of these battles Zoroaster was killed, but his faith lived on. To spread the Zoroastrian faith, missionaries carried its message throughout the land until it became a national religion. Later it spread beyond the border of Persia, and in time it interpenetrated Hebraic and Hellenistic thought, but it is difficult to trace any direct influence upon these later intellectual developments.
The conflict with Rome and the struggle over Armenia had been hightened by the issue of Christianity. The Iranians were Zoroastrians. In fact, by instituting the beliefs of the cult associated with Ahura Mazda as a state religion, the Sasanians had become militant Zoroastrians. As expressed under the regime in the form of orthodox Zoroastrian dualism, the good spirit of light Ohrmizd (Ahura Mazda), was opposed to the demon of evil, Ahriman. Ahriman was associated with subordinate angels, but these enjoyed a position in the official expression of the faith far inferior to that of the holy element of fire. The reverence for fire has given erroneous cause for the belief that the Zoroastrians were fire worshippers. By their insistence upon the orthodox form of dualistic Zoroastrianism and their persecution of heresies and even mildly heterodox cults, the Sasanians were responsible for the increasing formalization of the faith, for which the tending of the holy fires became one of the most highly prescribed rituals.
Many once-domed structures of the Sasanian era that survived have been labeled 'fire-temples' without any justification. Some of the surviving buildings could possibly be associated with secular pavilions or parts of palaces. The fire-temple designation has been applied simply because the tending of the holy fires appears to outsiders as the most exotic aspect of the religion. The Muslim conquerors of the seventh century AD were amazed by the numerous fire-temples, and some of these buildings were converted into mosques, which became the most distinctive architectural aspect of later Iranian architecture.
There were three categories of fires corresponding to the three original castes of society: priests, warrior, and commoners. In addition, there was the king's own royal fire. This was lit at the beginning of his reign and carried around on a portable altar. The fire is illustrated on the reverse of the silver coins. It is also known that there were prescribed rituals for the renewal of the strength of the fire, from parent fire down to that in the private home. Materials used were specially purified for the purpose and, after being brought out into the open for a ceremony, the fire was returned to its sanctuary for further purification. Unfortunately, it is difficult to disentangle the ancient Zoroastrian practices from the way in which customs have developed within the post-Islamic Zoroastrian communities, which are represented in their largest numbers by the Parsees of India. In Sasanian times various popular and philosophic believes including magic and demonology were also held by the people, and these attitudes survived in spite of the increasing formalistic expression of the religion and the growth of a rigid caste system for the priesthood.
The priest Kartir, who became prominent under Shapur I, was probably responsible for the most extreme orthodox practices. His inscriptions on the rock relief of Shapur and Bahram II attest to the fact that he was one of the most important figures in the empire, playing an important political, as well as religious role. The attitudes of Kartir were not original, for Ardashir had destroyed pagan monuments and had established fire-temples in their places before Shapur I. But Kartir was particularly zealous in that endeavor. He attacked Jews, Buddhists, Hindus, Manichaeans and Christians alike during the reign of five kings, from Shapur I to Narseh.
It is generally thought that Narseh was reasonably tolerant of the Manichaean faith. Mani had preached a more syncretistic religion, combining both Christian and Zoroastrian beliefs, based on universalism put forward by other gnostic sects. After preaching in India, Mani returned to Iran, where his ideas received considerable acceptance, even amongst members of the royal family. But he encountered the zeal of Kartir, who pronounced him as heretic, and Mani was executed. After his death, Manichaeism spread into Central Asia, even reached China. In spite of his official tolerance, Manichaean beliefs continued to be regarded as heretical by the clergy. The strict attitudes of Kartir were later relaxed, but not overthrown.
Later Islamic scribes wrote of various Zoroastrian sects, though modern commentators argue that it was a question of different schools of philosophical thought rather than different sects as such. The greatest conflict within Zoroastrianism itself involved the interpretation of the meaning of evil. The Mazdakists believed in the strictly separate origin of good and evil (Ohrmizd and Ahnrima respectively; others, called Zurvanites, believed that Ohrmizd and Ahriman had their origins from Zurvan, or 'infinite time'. Zurvangte tendencies were widespread in Sasanian times, and some commentators have argued that it actually became the dominant form of Zoroastrianism towards the end of the empire. It does seem that the largely pessimistic attitudes of the Zurvanites were particularly appropriate in that age of speculative gnosticism. But whether it came to dominate Zoroastrianism or not, it is clear that Zurvanism was eliminated as a philosophy amongst the surviving members of the Zoroastrian community, after the conquest of Islam.
A major heretical faith of the fourth century AD was Christianity. According to the Syrian sources, Christianity had gained a foothold in the Aramaic-speaking territories of the Tigris and Euphrates as early as the Parthian period. It is likely that the settlement of prisoners from Antioch may have helped encourage the spread of Christianity in western Iran. In AD 274, King Tiridates of Armenia adopted Christianity. But so long as Rome remained pagan, the Christians experienced no greater persecution than the followers of any other heterodox faith. However, the conversion of emperor 'Constantine' and the adoption by Rome as the official Christian faith, Christians in Iran were identified with one of the country's major political enemies. Persecution naturally followed. The first great purge occurred in AD 339, under Shapur II, who was vigorous in the pursuit of orthodox Zoroastrianism. He was responsible for ordering the first compilation of the religious writings of the Avesta. Christians feared a little better under his successors. Yazdigird I (AD 339-421) was extremely tolerant of both Jews and Christians; and, because of his inability to make headway against the Romans, Bahram V was forced to conclude a peace in which he granted freedom of worship to the Christians. But the most important change came in AD 483, when the Christian council Iran officially adopted the Nestorian doctrine as its dogma. Henceforth, the Christians of Iran did not always need to be identified completely with the power of Rome.