Before the Muslim conquest of Iran, the Persian people were predominantly Zoroastrian. The historian al-Masudi, a Baghdad-born Arab, who wrote a comprehensive treatise on history and geography in about 956, records that after the conquest:
"Zoroastrianism, for the time being, continued to exist in many parts of Iran. Not only in countries that came relatively late under Muslim sway (e.g Tabaristan) but also in those regions which early had become provinces of the Muslim empire. In almost all the Iranian provinces, according to Al Masudi, fire temples were to be found – the Madjus he says, venerate many fire temples in Iraq, Fars, Kirman, Sistan, Khurasan, Tabaristan, al Djibal, Azerbaijan and Arran."
This general statement of al-Masudi is fully supported by the medieval geographers who make mention of fire temples in most of the Iranian towns.
Also, Islam was readily accepted by Zoroastrians who were employed in industrial and artisan positions because, according to Zoroastrian dogma, such occupations that involved defiling fire made them impure. Moreover, Muslim missionaries did not encounter difficulty in explaining Islamic tenets to Zoroastrians, as there were many similarities between the two faiths. According to Thomas Walker Arnold, for the Persian, he would meet Ahura Mazda and Ahriman under the names of Allah and Iblis.[12] At times, Muslim leaders, in their effort to win converts, encouraged attendance at Muslim prayer with promises of money and allowed the Quran to be recited in Persian instead of Arabic so that it would be intelligible to all. Later, the Samanids, whose roots stemmed from Zoroastrian theocratic nobility, propagated Sunni Islam and Perso-Islamic culture deep into the heart of Central Asia. The first complete translation of the Quran into the Persian language occurred during the reign of the Samanids in the 9th century.
The "conversion curve" by Richard Bulliet highlights a relatively low conversion rate of non-Arab subjects during the Arab-centric Umayyad period, estimated at 10%. In contrast, during the more politically multicultural Abbasid period, the Muslim population increased significantly, from approximately 40% in the mid-9th century to nearly 80% by the end of the 11th century.
