Islam in Iran

The Arab conquest of Iran, which culminated in the fall of the Sasanian Empire to the nascent Rashidun Caliphate, brought about a monumental change in Iranian society by purging Zoroastrianism, which had been Iran’s official and majority religion since the time of the Achaemenid Empire. Since the Rashidun invasion, Islam (in any form) has consistently held the status of Iran's official religion except for a short period in the 13th century, when the Mongol invasions and conquests destroyed the Abbasid Caliphate and smaller Islamic realms before resulting in the establishment of the Ilkhanate. The process by which Iranian society became integrated into the Muslim world took place over many centuries, with nobility and city-dwellers being among the first to convert, in spite of notable periods of resistance, while the peasantry and the dehqans (land-owning magnates) took longer to do so. Around the 10th century, most Persians had become Muslims.

Between the 7th century and the 15th century, Sunni Islam was the dominant sect in Iran, and Iranian academics of this period contributed greatly to the Islamic Golden Age. In the 16th century, the newly enthroned Safavid dynasty initiated a massive campaign to install Shia Islam as Iran's official sect, aggressively proselytizing the faith and forcibly converting the Iranian populace. The Safavids' actions triggered tensions with the neighbouring Sunni-majority Ottoman Empire, in part due to the flight of non-Shia refugees from Iran. It is estimated that by the mid-17th century, Iran had become a Shia-majority nation. Over the following centuries, with the state-fostered rise of an Iran-based Shia clergy, a synthesis was formed between Iranian culture and Shia Islam that marked each indelibly with the tincture of the other. Later, under the Pahlavi dynasty, Islamic influence on Iranian society was rolled back in order to assert a new Iranian national identity—one that focused on pre-Islamic Iran by shedding more light on Zoroastrian tradition and other aspects of ancient Iranian society, particularly during the Achaemenid era. However, in 1979, the Islamic Revolution brought about yet another monumental change by ending the historic Iranian monarchy and replacing it with an Islamic republic.

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% of Iranians identify as Muslims. 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.

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, 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."