The Muslim conquest of Persia (637651) led to the end of the Sasanian Empire and the eventual decline of the Zoroastrian religion in Iran (Persia). However, the achievements of the previous Persian civilizations were not lost, but were to a great extent absorbed by the new Islamic polity. Islam has been the official religion of Iran since then, except for a short duration after the Mongol raids and establishment of Ilkhanate. Iran became an Islamic republic after the Islamic Revolution of 1979 which ended the Persian monarchy.
Before the Islamic conquest, the Persians had been mainly Zoroastrian; however, there were also large and thriving Christian and Jewish communities, especially in the territories of at that time northwestern, western, and southern Iran, mainly Caucasian Albania, Asristn, Persian Armenia, and Caucasian Iberia. There was a slow but steady movement of the population toward Islam. When Islam was introduced to Iranians, the nobility and city-dwellers were the first to convert, Islam spread more slowly among the peasantry and the dehqans, or landed gentry. By the late 11th century, the majority of Persians had become Muslim, at least nominally.
Iran's population is about 82,000,000 people, and Islam is the religion of 99.4% of Iranians.  Nearly 90% of Iranian Muslims are Shi'a and about 10% are Sunni. Most Sunnis in Iran are Kurds, Achomis, Turkmens, and Baluchs, living in the northwest, northeast, south, and southeast. Almost all of Iranian Shi'as are Twelvers.
Though Iran is known today as a stronghold of the Shi'a Muslim faith, it did not become so until much later, around the 15th century. The Safavid dynasty made Shi'a Islam the official state religion in the early sixteenth century and aggressively proselytized on its behalf. It is also believed that by the mid-seventeenth century most people in Iran, Iraq and the territory of the contemporary neighboring Republic of Azerbaijan had become Shi'as, an affiliation that has continued. Over the following centuries, with the state-fostered rise of a Persian-based Shi'ite clergy, a synthesis was formed between Persian culture and Shi'ite Islam that marked each indelibly with the tincture of the other.