The Muslim conquest of Persia () led to the end of the Sasanian Empire and triggered the decline of Zoroastrianism among the Iranian peoples due to large-scale persecution by Arab Muslims under the newly-arrived Rashidun Caliphate. Since its establishment after the 7th-century conquest, Islam has remained the official religion of Iran (also known as "Persia") except for during a short period after the Mongol invasions and subsequent establishment of the Ilkhanate in the 13th century. The 1979 Islamic Revolution brought an end to the historic Persian monarchy, after which Iran emerged as an Islamic republic. Before the Muslim conquest, mainland Iranians primarily adhered to the Iranian religion of Zoroastrianism; there were also large and thriving Jewish and Christian communities, especially in the territories of northwestern, western, and southern Iran—mainly Caucasian Albania, Asoristan, Persian Armenia, and Caucasian Iberia. A significant number of Iranian peoples also adhered to Buddhism in what was then eastern Iran, such as the regions of Bactria and Sogdia. Following the Muslim conquest, there was a slow but steady movement of the population toward Islam, despite notable resistance. When Islam was introduced to Iranians, the nobility and city-dwellers were among the first to convert; Islam spread more slowly among the peasantry and the dehqans, or land-owning magnates. By the 10th century, the majority of Persians had become Muslims. However, the achievements of the previous Persian civilizations were not lost, but were to a great extent absorbed by the new Islamic polities. According to the Islamic Republic's official survey, almost all of Iran's 82,000,000 people are Muslims, with 90 percent adhering to Shia Islam, and a majority of that Shia populace following the Twelver branch. Another 10 percent adhere to Sunni Islam, most of them being Kurds, Achomis, Turkmens and Baloch, living in the northwest, northeast, south and southeast, respectively. A 2020 survey by the World Values Survey found that 96.6 percent of Iranians believe in Islam. However, according to another 2020 online survey by the GAMAAN, which provided greater anonymity, there has been a sharp decline in religiosity in Iran, and only 40 percent of Iranians who took part of the online survey identified as Muslims. Something The Wall Street Journal called a "crisis of faith". Islam in Iran can be categorised into two periods: Sunni Islam as a majority from the 7th century to the 15th century, and Shia Islam as a majority from the 16th century