By: Mir M.Hosseini
During Alexander's conquest of Persia, all copies of the holy book Avesta was burned, and the scientific sections that the Greeks could use were dispersed among themselves. Under the reign of King Valax of the Arsacid Dynasty, an attempt was made to restore the Avesta but Hellenism was practiced despite Iranians' tendency towards monotheism.
During the Sassanid Empire, Ardeshir 1st ordered Tansar, his high priest, to finish the work that King Valax had started. On Dec, 21, 233, the day after Yalda celebrations Ardeshir announced Zoroastrianism as the official state religion of Iran. He ordered Zoroastrian Fire Temples to be revived. Ardeshir thus established a strong political centralization and organized state sponsorship of Zoroastrianism.
Zoroastrianism had existed in the Parthian Empire and according to tradition—its sacred literature had been collated during that era. Similarly, the Sassanids traced their heritage to the Temple of Anahita at Staxr, where Ardeshir's grandfather had been a dignitary. Under Ardeshir however, Zoroastrianism was promoted and regulated by the state, one based on the ideological principle of divinely granted and indisputable authority. The Sassanian Dynasty built fire temples and, under royal direction, an orthodox version of the Avesta was compiled by Tansar, and it was during the early period that the texts as they exist today were written down (until then they had been orally transmitted).
During the reign of his son Shapour 1st, priests were sent to locate the scientific text portions of the Avesta that were in the possession of the Greeks. Under Shapour 2nd, Azarbad Mahrespandan revised the canon to ensure its orthodox character, while under Khosrau 1st, the Avesta was translated into Pahlavi.
Zoroastrianism was marginalized by Islam from the 7th century onwards with the decline of the Sassanid Empire. In time, a tradition evolved by which Islam was made to appear as a partly Iranian religion. One example of this was that Imam Hossein the son of the fourth caliph Imam Ali and grandson of Prophet Mohammad had married a captive Sassanid princess named Shahrbanou who was said to have borne Hossein a son, the historical fourth Shia Imam, who claimed that the caliphate rightly belonged to him and his descendants, and that the Umayyad had wrongfully wrested it from him. The alleged descent from the Sassanid house counterbalanced the Arab nationalism of the Umayyad, and the Iranian national association with a Zoroastrian past was disarmed.
Today the Iranian constitution states that Islam is the official state religion, and the doctrine followed is that of Jafari (Twelver) Shiism. The constitution provides that "other Islamic denominations are to be accorded full respect," while the country's pre-Islamic religious groups -- Zoroastrians, Christians, and Jews -- are recognized as "protected" religious minorities.