Cyrus took Babylon in 539 BCE, and the Achaemenid empire reached from the Indus to the Aegean, ruled with a light hand in religious matters; the Hebrew Bible calls Cyrus the anointed of the Lord for permitting the return to Jerusalem. Whether the Achaemenid kings were Zoroastrian in any strict sense is argued, since their inscriptions name Ahura Mazda but say nothing of Zarathushtra.
Alexander's conquest in 330 BCE ended the empire, and Zoroastrian tradition remembers him as the destroyer of scriptures. Under the Sasanians, from the third century of the common era, the religion became an organised state church with a priestly hierarchy, a canon, and at times the persecution of Manichaeans and Christians. Manichaeism itself arose in that setting, as did the Mazdakite movement.
The Arab conquest of the seventh century made Islam the religion of Iran over several centuries rather than immediately. A community moved to Gujarat and became the Parsis, and John Hinnells's study traces the further migrations that made Zoroastrianism a global diaspora now numbering well under two hundred thousand. Iranian religious history did not stop there. Iran became the principal home of Twelver Shia Islam, and the Babi and Bahai movements began there in the nineteenth century, the restrictions on Bahais being reported to the United Nations to this day.