Islam entered Iran 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.

Iran, also known as Persia and officially the Islamic Republic of Iran (IRI), is a country in West Asia. It is bordered by Iraq to the west and Turkey to the northwest, Azerbaijan, Armenia, the Caspian Sea and Turkmenistan to the north, Afghanistan to the east, Pakistan to the southeast, the Gulf of Oman and the Persian Gulf to the south. With a mostly Persian-ethnic population of almost 90 million in an area of 1,648,195 km2, Iran ranks 17th globally in both geographic size and population. It is the sixth-largest country entirely in Asia, the second-largest in West Asia, and one of the world's most mountainous countries. Officially an Islamic republic, Iran has a Muslim-majority population. The country is divided into five regions with 31 provinces. The nation's capital and most populous city is Tehran, with around 16 million people in its metropolitan area. Other major cities include Mashhad, Isfahan, Karaj, and Shiraz.