The main difference originated from the question of succession after the death of the Prophet Muhammad and is related to differing views about appropriate leadership for the Muslim community. Shi’as believe that succession to the spiritual and political rule of the Muslim community lies only with the family and certain descendants of the Prophet Muhammad. Sunnis believe that the Muslim community is free to choose whoever appears to be the most qualified person. Shi’as believe that God chose Muhammad’s cousin Ali, who was married to his daughter Fatima, to be the Prophet Muhammad’s successor, and that Muhammad formally announced this before his death. Shi’as also view Ali as the first in a line of Imams, or preeminent religious leaders, whom they regard as the spiritual and political successors to Muhammad. In contrast, Sunnis believe that Muhammad did not appoint any particular person to be his spiritual or political successor; and that the senior Muslims chose Abu Bakr As-Sideeq to be the successor (Caliph).

Other theological differences exist between the two, many stemming from their divergent views of leadership, the role of the Imams, and resulting variations in beliefs and practices. However, both mainstream Sunnis and Shi’as share the core beliefs of Islam—the oneness of God and the prophethood of Muhammad—and adhere to the five pillars.