Iran is overwhelmingly Shia Muslim, around 90 to 95 percent Twelver Shia per most estimates including Pew Research's Religious Composition of the World, 2010-2020, with Sunni Muslim minorities among Kurds, Baluchis and Turkmen, and small recognized Christian, Jewish and Zoroastrian communities. Zoroastrianism, founded by the prophet Zoroaster and centered on the god Ahura Mazda, was Persia's dominant religion for over a thousand years under the Achaemenid, Parthian and Sasanian empires before the seventh-century Arab Muslim conquest, and a small Zoroastrian community survives in Iran today, guaranteed reserved parliamentary representation alongside Jewish, Armenian Christian and Assyrian Christian minorities.
Shia Islam became Iran's dominant faith specifically through the Safavid dynasty's imposition of Twelver Shia orthodoxy as state religion from 1501, a pivotal transformation that set Iran apart religiously from the mostly Sunni Ottoman and Central Asian worlds surrounding it. Qom and Mashhad rank among Shia Islam's most important centers of religious scholarship and pilgrimage, the latter holding the shrine of Imam Reza, the eighth Shia Imam, and Iranian Shia clergy, organized in a hierarchy of religious scholarship, would later provide the theological and institutional foundation for the 1979 Islamic Revolution.