Tomb of Xerxes I (d. 465), Naqsh-e Rustam

Ancient Iran was a land of many religions. Its primary tradition, Zoroastrianism, enjoyed state sponsorship under three of its most powerful dynasties - the Achaemenids, Parthians, and Sasanians - each of whom linked their authority to that of the priestly class of the magi. Alongside Zoroastrianism, numerous other religions found a foothold in Ancient Iran, from Mesopotamian rituals adopted by the Elamites to Jewish, Christian, and Manichaean communities under the Sasanian Empire.

The history of Judaism in Ancient Iran offers a particularly interesting case. When Cyrus defeated the Babylonians in 538 BC, he issued a decree allowing the inhabitants of the empire, including the deportees from Judea, to rebuild their temples. It has been argued that this act of good faith inspired a large Jewish population to remain in Iranian territory, where they would later produce some of medieval Judaism's most important intellectual work.

Statue of Heracles at Behistun, ca. 2nd century BC

Under Seleucid and Parthian rule, additional religious traditions, both indigenous and non-native, took hold in Iran through invasion, migration, and commerce. Alexander of Macedon and the ensuing Seleucid dynasty popularized aspects of the Greek pantheon among the ruling class, and syncretism created interesting combinations of Zoroastrian, Hellenic, and other traditions, such as the blending of the Greek hero Heracles with the Iranian god Verethragna. Syriac Christians founded communities, primarily in Mesopotamia and western Iran. Buddhists emerged in the eastern Iranian sphere as well.  

Although Sasanian emperors strongly favored Zoroastrianism and held less favorable views of religious diversity than the Parthians, they nonetheless generally continued a policy of tolerance. When some herbeds (high Zoroastrian priests) petitioned the 6th century emperor Hormizd IV to persecute the growing number of Christians in Persia, he is said to have replied, "Just as our royal throne cannot stand on its two front legs without the two back ones, our kingdom cannot stand or endure firmly if we cause the Christians and adherents of other faiths, who differ in belief from ourselves, to become hostile to us" (al-Tabari). The king's response illustrates how Zoroastrianism, while dominant, never achieved exclusivity over the other religious traditions of Ancient Iran.