Iran wasn’t a monolithic Zoroastrian state when it fell to Arab armies. Iran was a huge multi-religious multi-ethnic empire. While Zoroastrianism was the dominant religion, Christianity, Judaism, and Manicheanism were large and powerful religions.

In Part II of Why Did Iran Embrace Islam I deal with the state of Zoroastrianism in Iran in the 7th century and the 8th, the position of Zoroastrian clergy, and the hostility of Zoroastrians to the avaricious priestly class.