Why did Iran embrace Islam?
Facts and fiction: Part I
Iranians were the earliest converts to Islam outside of the Arabian Peninsula, and our conversion in large numbers began as soon as the Arab armies reached and overran the Persian plateau (a part of western Iran and southern Iraq now). Despite resistance from elements of the Zoroastrian clergy, Jews and Christians, the vast majority of Iranians became and have remained Muslims. However, northern Iran remained out of the Muslim caliphate (empire), remained Zoroastrian and Christian until its conversions to Zaidi Shi’a Islam and Hanbali in the 10th century —nearly 300 years after the Arab conquest of central and southern Iran and the Khorasan.
Conversions in the early periods were not regarded as a formal or ritual act and, in the absence of religious organisations and clergy, conversions couldn't be verified. Very few officials (i.e., Ashras Abd-Allah at Samarkand), insisted on proof of a full, formal adherence to Islam, including circumcision, before accepting a conversion as valid1.
In the early days, most people made nominal conversions to Islam without full acceptance or understanding of the rituals or doctrinal obligations involved. This quickly led to many popular, syncretistic sects in early Islamic Iran, and heresiographers and tax collectors were confused about which should be regarded as Muslim. Some sects were classified as Muslim in one province but not in another. Some sects successfully bribed tax collectors to classify them as Muslims2.
The Korramdiniya used mosques and the Koran but did not adhere to any of the ritual or legal requirements3. The Kurdish group (Kurdanaye) combined aspects of Zoroastrianism, Christianity, and Islam but declared themselves Muslims4.
Recent research has established a general chronological framework for the process of conversion of Iranians to Islam. From a study of the probable dates of individual conversions based on genealogies in biographical dictionaries, it's clear that there was a gradual and limited conversion of Persians down to the end of the Umayyad period (750 AD), followed by a rapid increase in the number of conversions after the ʿAbbasid revolution by non-Arab Muslims and their Shi'a allies.
The Persian Empire (Sasanian Iran) fell in 651 AD and the Abbasid rule ended in Khorasan Iran in 950 AD. By the end of these 200 years, Iran had become 80% Muslim. It took 200 years for Iran to become 80% Muslim, but on the other hand, its arch-enemy, the Roman Empire, had become 80% Christian in half that time!
Coming next (Part II): Why did the Zoroastrian population turn against the magi (Zoroastrian clergy)?
Haug, R. (2019). The Eastern Frontier: Limits of Empire in Late Antique and Early Medieval Central Asia. India: Bloomsbury Publishing.
Anderson, J. (1951). Conversion and the Poll Tax in Early Islam. By Daniel C. DennettJr. Harvard Historical Monograph No. XXII, 1950. pp. xi, 136, including bibliography and index. 16s. Journal of the Royal Asiatic Society,83(3-4), 220-220. doi:10.1017/S0035869X00104952
Baḡdādī, Farq, ed. ʿAbd-al-Ḥamīd, p. 269; Ebn Ḥawqal, tr. Kramers, p. 364
Michael the Syrian, p. 50; Bar Hebraeus, pp. 131-32