Part of this was originally written on Quora on July 12, 2020 - Were Persians forced into accepting Islam? Did Islam borrow its Hadith & Quran Teachings from Zoroastrianism & Persian?
What is commonly understood, in the non-Muslim world, in particular the Western countries, that the followers of Zoroastrianism were converted, or must have converted, by force after the Islamic Arab conquest of Persia. After scrutinizing the history of Islam in Persia, one cannot believe in such allegations. In fact there is ample evidence that despite the overthrow of the Zoroastrianism following Persian Dynasty, Islam was spread through peaceful and other non-violent means among Zoroastrians as well as other faiths.
In Pre-Islamic Iran (Persian) the Zoroastrian priests enjoyed an enormous influence in the state, due to their relations with ruling dynasty and with their authority persecuted people’s of other faiths such as Sabians, Christians, Jews, Buddhists. This created a hatred for established religion and the dynasty and so the people of these faiths viewed the Muslim Arab conquest of Persian as means of liberation from their persecution. Islam tolerated their religion and practices for light tribute (jizya, a tax on non-Muslims) that gave them exemption from military service and their property, family and lives were under the protection of Islamic state. In fact the fire worshipers of Zoroastrian faith were also allowed to practice their faith and live in harmony with others and were even given the title of Ahl al-Kitab (people of the book) like Jews & Christians as taught in the prophetic hadith: “Follow the same sunna with them that you follow with the people of the Book” (as recorded in Book 17, Hadith 43 of Muwatta Malik) The term ahl al-kitab is generally translated in English as “People of the Book” and understood as referring to Jews and Christians in many instances in the Quran (depending upon context). But from a broader perspective the term includes all those nations and people who have ever received a divine revelation from Allah ﷻ. The famous Muslim of 20th century Muhammad Asad translates the term as “Followers of the earlier revelations” (such as in Al Quran 98:1) and used it throughout his English Translation. This means all those nations who ever received a divine book from Allah ﷻ which would include Hindus, Buddhists, Zoroastrians may be even Chinese or any nations who claim to have ever received a divine book. Hence we notice that after the fall of Sassanid Empire, the central Zoroastrian religious authority had no support to rely on and the followers of this creed would find the transition to Islam a simple and easy one, owing to the numerous points of similarity in the old creed and the new such as Ahuramazda and Ahriman were qual to Allah ﷻ and Iblis, the accursed; the creation of the world in six periods; the angels and the demons; the story of the primitive innocence of man; the resurrection of the body and the doctrine of heaven and hell. Islam enjoined to pray five times a day just as they had been by the Avesta.
In his Preaching of Islam, Professor T. W. Arnold argues that the political and national sympathies of the conquered Persians were also enlisted on behalf of the new religion through the marriage of Husayn (alaihi salaam), the son of the 4th rightly guided Caliph Ali (alaihi salaam) and the grandson of Prophet Muhammad ﷺ, with Shahbanu, one of the daughters of Yazdagird, the last monarch of the Sassanid dynasty. In the descendants of Shahbanu and Husayn the Persians saw the heirs of their ancient kings, and this may also explain the devotion of many Persians towards Ali ibn Abi Talib (a.s) and Shiaism. The widespread conversion to Islam among Persians could not have been forcibly because the fact that many fire worshiping communities and their places of worship exist in Iran, till this day. In the 10 Century CE, three centuries after the conquest of the country, fire-temples were to be found in Iraq, Fars, Kirman, Sijistan, Khurasan, Jibal, Azharbaijan and Arran, i.e. in almost every province of Persia. Al-Sharastani also (writing as late as the twelfth century), makes mention of a fire-temple at Isfiniya, in the neighbourhood of Baghdad itself, the capital of Abbasid Caliphate. Another factor that led towards many Zoroastrians accepting Islam may have been the class system that was part of their religion with priesthood on the top and the artisans, laborers, the lowest class, at the bottom of the societal hierarchy. Islam was the faith that made the lower classes equal in faith and brotherhood with other Muslims.
Professor Arnold also adds how acceptance of Islam by Persian nobles and ruling class spread Islam. About close of the 8th Century CE, Saman, a noble of Balkh, having received assistance from Asad bin Abdallah, the governor of Khurasan, renounced Zoroastrianism, embraced Islam and named his son Asad after his protector : it is from this convert that dynasty of the Samanids (CE 874-999) took its name. About the beginning of the 9th Century CE, Karam bin Shahriyar was the first king of the Qabusiyyah dynasty who became a Muslim, and in 873 CE a large number of fire-worshippers were converted to Islam in Daylam through the influence of Nasir al-Haqq Abu Muhammad. In the following century, about CE 912, Hasan bin Ali, a Zaydi Shia revolutionary who on the southern shore of the Caspian Sea, who is said to have been invited the inhabitants of Tabaristan and Daylam, who were Magians, to accept Islam; many of them responded to his call. In the light of the given facts, it is surely impossible to attribute the decay of Zoroastrianism entirely to violent conversions by Muslim conquerors.
Muhammad Asad - Message of the Quran - Chapter 98