Islam in Iran

Islam entered Iran (or "Persia") with the Muslim conquest (637–651) and led to the end of the Sasanian Empire and the eventual decline of the Zoroastrian religion in Iran (Persia). Since its establishment after the 7th-century conquest, Islam has remained the state religion of Iran except for during a short period after the Mongol invasions and subsequent establishment of the Ilkhanate in the 13th century. Following the Muslim conquest, there was a slow but steady movement of the population toward Islam, despite notable resistance, with nobility and city-dwellers being the first to convert, and the peasantry and the dehqans, or land-owning magnates slower to do so. By the 10th century, the majority of Persians had become Muslims.

While Sunni Islam dominated Iran from the 7th century to the 15th century, in the 16th century the Safavid dynasty made Shi'ism the state religion, and aggressively proselytized the faith by forced conversion. It is believed that by the mid-17th century most people in Iran, (and some surrounding areas), had changed their religion from Sunni to Shia, an affiliation that has continued. 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. The 1979 Islamic Revolution brought yet another change, ending the historic Persian monarchy, and establishing Iran as a theocratic Islamic republic.

According to the Islamic Republic's 2016 census almost all of Iran's 89 million as of 2023 population are Muslims with 99% reporting that they are Muslim. with 88% adhering to Shia Islam, which an overwhelming majority of those following the Twelver branch of Shia Islam. Approximately 12% adhere to Sunni Islam, most of them being ethnic minorities such as the (Arabs, Kurds, Achomi Persians, Turkmens, and Baloch).

According to the 2020 Wave 7 World Values Survey 96% of those surveyed in Iran followed and Identified with Islam while according to another 2020 online survey by GAMAAN, there has been a sharp decline in religiosity in the survey, and only 40% of Iranians who took part of the online survey identified as Muslims. Subsequent GAMAAN surveys in 2022 showed that, depending on how the question is asked, 38% to 56% identify as Shia Muslims, 5% as Sunni Muslims, and that roughly a quarter of them were susceptible to a form of deism (belief in God without identifying as religious). In all of the GAMAAN's surveys, 7-10% identified as atheists. Moreover, the online surveys offered respondents a unique opportunity to express themselves, leading to fluctuating numbers of self-identified Zoroastrians.