Iran's official religion is Shia Islam, which is one of the two major branches of Islam. Shia Islam is the largest branch of Islam in Iran, and it is the official state religion. Shia Muslims believe that the Prophet Muhammad's son-in-law, Ali, was the rightful successor to the Prophet Muhammad. This belief is based on the teachings of the Quran and the Hadith. Shia Muslims also believe in the concept of Imamah, which is the belief that certain individuals have been chosen by God to lead the Muslim community.