The differences between Sunni and Shi’i Muslims stem from questions about Muhammad’s successors. Shi’i groups historically argued that Ali was Muhammad’s successor, and continued recognizing successors from within the Prophet Muhammad’s family. Sunni groups historically argued for Abu Bakr al-Siddiq as Muhammad’s successor, and continued to select leaders from across the community.
The Shi’at Ali (“the party of Ali,” for which "Shi’ah" is an abbreviation and from which Shi’i comes) considered the descendants of the Prophet to be the only legitimate successors to Muhammad as leaders of the ummah. They believed that Muhammad’s barakah (spiritual grace or power) and special knowledge to interpret the Qur’an had been given to Ali, the son-in-law and cousin of the Prophet. The leadership was passed on by the designation of a successor (known as the Imam) within the Prophet’s family. According to the Shi’ah, a community without the direct revelation of a prophet must always have an imam, who will maintain the revelation and guide the community in applying it to new situations.
The majority of Shi’is, known as “Twelvers,” recognize a line of twelve Imams, the last of whom disappeared in the late 9th century CE. Believed to be the Mahdi (rightly-guided one or “Messiah”), he is expected to return in the Last Days to establish truth and justice. Other Shi’i groups, such as the Zaydis and Ismailis, trace the succession differently: the Ismailis believe that the last Imam was Isma’il; the Zaydis believe that the Imam must be descended from Fatima and Ali, the Prophet’s daughter and son-in-law, but he need not be the son of an Imam.
The lives and the sufferings of the Imams are commemorated in story and ritual, as can be seen in the “passion plays” given on the 10th day of Muharram, or the Day of Ashura, to recall the martyrdom of Husayn, grandson of the Prophet, in 680 CE. The words of the Imams are also a source of law for Shi’i Muslims, in addition to the Qur’an and Sunnah of the Prophet.
The majority of the early community chose Abu Bakr al-Siddiq to be the caliph of Muhammad because of his skills, piety and close friendship with the Prophet. They insisted that Muhammad had given his authority to the whole community, which could then choose its own leaders. Religious authority was gained by following the Qur’an, the Sunnah of the Prophet, and ijma, the consensus of Muslims. This majority community thus became known as “the people of the Sunnah,” or “Sunnis.” The Sunni caliphs expanded the borders of the early Muslim empire; the Umayyad dynasty assumed the caliphate, after 661 CE and ruled from Damascus. Opposition to the Umayyads took the form of Shi’i, Sufi, Shar’i (law-oriented) and other movements.