Twelver Shi'a Islam is the official religion of Iran. Islam was introduced through the Arab conquest of Iran in the 7th century, which led to the fall of the Sasanian Empire and the gradual conversion of the Iranian population over several centuries. By the 10th century, most Persians had become Muslims, and Iranian dynasties and scholars played a major role in the development of Islamic civilization during the Islamic Golden Age.
Between the 7th and 15th centuries, Sunni Islam was the dominant sect in Iran. In the early 16th century, the Safavid dynasty established Twelver Shia Islam as the state religion through state-sponsored conversion campaigns, transforming Iran into a predominantly Shia country. Over the following centuries, Shia Islam became closely intertwined with Iranian identity and political culture."The Origins Of The Shiite-Sunni Split". NPR.org. Retrieved 14 February 2021.John Obert Voll (1994). Islam, continuity and change in the modern world. Internet Archive. Syracuse University Press. .
Since the Iranian Revolution of 1979, Iran has been governed as an Islamic republic based on the doctrine of Guardianship of the Islamic Jurist (velayat-e faqih), with Shia Islam and the Ja'fari school designated as the official religion and basis of the legal system.Iranian Government Constitution , English Text, English Text Iran is today a predominantly Shia Muslim country, with Sunni Muslim minorities and smaller non-Muslim communities.
Western estimates of adherence According to an October 2024 update to Britannica.com by scholars at the Universities of Cambridge and Utah, as of the date of their source data, Muslims accounted for 99.6% of the total population of Iran and comparable older numbers and somewhat discrepant more recent numbers, at CIA.gov—with the "vast majority... [stated as being] of the Ithnā ʿAsharī, or Twelver, Shiʿi branch".
The Iranian government's 2016 census purportedly presents 99% of the Iranian population as Muslim, and 80% of this figure is composed of Twelver Shias. Approximately 7% of Iranians are Sunnis. According to scholars at the Universities of Cambridge and Utah, the country's "Kurds and Turkmen are predominantly Sunni Muslims", with Iran's Arab population being split between "Sunni and Shiʿi" (Shia). Other sources note that this smaller percentage comprises the country's ethnic minorities, the Kurds, Turkmens, and Arabs, as well as Achomi Persians, Khorasani Persians, and Baloch.
According to the 2020 Wave 7 World Values Survey, 96.6% of Iranians identify as Muslims.
GAMAAN surveys However, a report by the Group for Analyzing and Measuring Attitudes in Iran (GAMAAN) in the same year showed a sharp decline in religiosity in the country, as only 40% of Iranian respondents identified as Muslims. Subsequent GAMAAN surveys in 2022 showed that, 38% to 56% identified as Shia Muslims, 5% identified as Sunni Muslims, and roughly a quarter of were susceptible to a form of deism—that is, belief in God without identifying as religious. In all GAMAAN surveys, 7% to 10% of Iranian respondents identified as atheists.
However, even for opponents of the Islamic Republic the GAMAAN surveys have been the object methodological criticisms, including online and snowball-sampling surveys, focus on the use of non-probability, self-selected samples recruited through social media, which critics argue introduce systematic bias and limit representativeness. Weighting procedures are considered insufficient to correct for these structural sampling biases. Additional concerns include coverage error stemming from unequal internet access, disproportionately excluding rural, elderly, and lower-income populations, as well as limited transparency regarding raw data, recruitment methods, and protections against duplicate or automated responses, which constrains independent verification and replicability.
History
Arab conquest of Iran
Arab Muslims conquered Iran in the time of Umar (637) and conquered it after several great battles. Yazdegerd III fled from one district to another Merv in 651. By 674, Arab Muslims had conquered Greater Khorasan (which included modern Iranian Khorasan province and modern Afghanistan, Transoxania).
As Bernard Lewis has quoted "These events have been variously seen in Iran: by some as a blessing, the advent of the true faith, the end of the age of ignorance and heathenism; by others as a humiliating national defeat, the conquest and subjugation of the country by foreign invaders. Both perceptions are of course valid, depending on one's angle of vision."
Under Umar and his immediate successors, the Arab conquerors attempted to maintain their political and cultural cohesion despite the attractions of the civilizations they had conquered. The Arabs were to settle in the garrison towns rather than on scattered estates. The new non-Muslim subjects, or dhimmi, were to pay a special tax, the jizya or poll tax, which was calculated per individual at varying rates for able bodied men of military age.
Iranians were among the very earliest converts to Islam, and their conversion in significant numbers began as soon as the Arab armies reached and overran the Persian plateau. Despite some resistance from elements of the Zoroastrian clergy and other ancient religions, the anti-Islamic policies of later conquerors like the Il-khanids, the impact of the Christian and secular West in modern times, and the attraction of new religious movements like Babism and the Baháʼí Faith (qq.v.), the vast majority of Iranians became and have remained Muslims. Today perhaps 98 percent of ethnic Iranians, including the population of Persia, are at least nominal Muslims. For such a fundamental, pervasive, and enduring cultural transformation, the phenomenon of Iranian conversions to Islam has received remarkably little scholarly attention.for an early and still worthwhile survey of the subject, see Arnold, pp. 209–20; for significant recent advances, see Bulliet, 1979a; idem, 1979b
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, Richard Bulliet has suggested that there was gradual and limited conversion of Persians down to the end of the Umayyad period (132/750), followed by a rapid increase in the number of conversions after the ʿAbbasid revolution, so that by the time when regional dynasties had been established in the east (ca. 338/950) 80 percent or more of Iranians had become Muslims. The data on which Bulliet's study was based limited the validity of this paradigm to generalizations about full, formal conversions in an urban environment. The situation in rural areas and individual regions may have been quite different, but the overall pattern is consistent with what can be deduced from traditional historical sources. Although in some areas, for example, Shiraz at the time of Moqaddasi's visit in about 375/985 (p. 429), there may still have been strong non-Muslim elements, it is reasonable to suppose that the Persian milieu as a whole became predominantly Islamic within the period of time suggested by Bulliet's research.
Islamization of Iran
Following the Abbasid revolution of 749–51, in which Iranian converts played a major role, the Caliphate's center of gravity moved to Mesopotamia and underwent significant Iranian influences. Accordingly, the Muslim population of Iran rose from approx. 40% in the mid 9th century to close to 100% by the end of the 11th century. 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.The preaching of Islam: a history of the propagation of the Muslim faith By Sir Thomas Walker Arnold, pg.170-180 Moreover, Muslim missionaries did not encounter difficulty in explaining Islamic tenets to Zoroastrians, as there were many similarities between the faiths. According to Thomas Walker Arnold, for the Persian, he would meet Ahura Mazda and Ahriman under the names of Allah and Iblis. Muslim leaders in their effort to win converts encouraged attendance at Muslim prayer, and allowed the Quran to be recited in Persian instead of Arabic so that it would be intelligible to all. The first complete translation of the Qur'an into Persian occurred during the reign of Samanids in the 9th century. Seyyed Hossein Nasr suggests that the rapid increase in conversion was aided by the Persian nationality of the rulers.Tobin 113-115Nasr, Hossein, Islam and the Plight of Modern Man
According to Bernard Lewis: "Iran was indeed Islamized, but it was not Arabized. Persians remained Persians. And after an interval of silence, Iran reemerged as a separate, different and distinctive element within Islam, eventually adding a new element even to Islam itself. Culturally, politically, and most remarkable of all even religiously, the Iranian contribution to this new Islamic civilization is of immense importance. The work of Iranians can be seen in every field of cultural endeavor, including Arabic poetry, to which poets of Iranian origin composing their poems in Arabic made a very significant contribution. In a sense, Iranian Islam is a second advent of Islam itself, a new Islam sometimes referred to as Islam-i Ajam. It was this Persian Islam, rather than the original Arab Islam, that was brought to new areas and new peoples: to the Turks, first in Central Asia and then in the Middle East in the country which came to be called Turkey, and India. The Ottoman Turks brought a form of Iranian civilization to the walls of Vienna..."
Iranian influence on Muslim civilization
The Islamization of Iran was to yield deep transformations within the cultural, scientific, and political structure of Iran's society: The blossoming of Persian literature, philosophy, medicine and art became major elements of the newly forming Muslim civilization. Inheriting a heritage of thousands of years of civilization, and being at the "crossroads of the major cultural highways",Caheb C., Cambridge History of Iran, Tribes, Cities and Social Organization, vol. 4, p305–328 contributed to Persia emerging as what culminated into the "Islamic Golden Age". During this period, hundreds of scholars and scientists vastly contributed to technology, science and medicine, later influencing the rise of European science during the Renaissance.Kühnel E., in Zeitschrift der deutschen morgenländischen Gesell, Vol. CVI (1956)
Iran was home to many of the most prominent figures of Islamic thought across both Shia and Sunni branches, including Hadith collectors like Shaikh Saduq, Shaikh Kulainy, Imam Bukhari, Imam Muslim and Hakim al-Nishaburi and theologians like Shaykh Tusi, Imam Ghazali, Imam Fakhr al-Razi and Al-Zamakhshari. Prominent Islamic physicians, astronomers, logicians, mathematicians, metaphysicians, philosophers and scientists like Al-Farabi, Avicenna, and Nasīr al-Dīn al-Tūsī were Iranian or lived in Persia. Iranians also played a prominent role in the development of Sufism, with notable Shaykhs like Rumi and Abdul-Qadir Gilani residing in Iran.
Ibn Khaldun narrates in his Muqaddimah:Translated by F. Rosenthal III, pp. 311–15, 271-4 [Arabic]; R.N. Frye (p.91)