The majority of both 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.
Historically, the 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. While both the majority of Sunnis and Shi’as assign special status to and revere the descendants and family of the Prophet Muhammad, 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. Specifically, 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 indicated this before his death. In support of their position Shi’as reference a sermon by the Prophet Muhammad shortly before his death at a place called Ghadir Khumm, in which he stated “to whomsoever I am mawla, Ali is also their mawla.” The point of contention is the meaning of the word mawla; the Sunni interpretation of mawla here is “friend,” whereas the Shi’a interpretation is “master,” which includes political leadership. 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 that the Muslim community was free to choose the most qualified person as ruler and that Muhammad did not appoint any particular person as his political successor, although he lauded his kinsmen, descendants, and companions as the spiritual heirs of his teachings.
This difference in interpretation affected not only political leadership but also the development of Islamic theology, as each group had different methods of exegetical approach to the Qur’an and different criteria for authenticating Hadith. The main differences between them today are their sources of knowledge and religious leadership. In addition to the Qur’an and Hadith, the Shi’as rely on the rulings of their Imams and resulting variations in beliefs and practices.