The Shi’at Ali (“the party of Ali,” for which Shi’a is an abbreviation and from which Shi’a 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 baraka (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. Shi’a Muslims believe that leadership was passed on by the designation of a successor (known as the Imam) within the Prophet’s family. According to the Shi’a, 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’as, known as “Twelvers,” recognize a line of twelve Imams, the last of whom disappeared in the late ninth 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 on earth. Other Shi’a groups, such as the Zaydis and Isma’ilis, trace the succession differently. The Isma’ilis trace their spiritual succession from Isma’il to the current, living Imam, His Highness Prince Karim Aga Khan who is the 49th Imam in direct lineal descent from the Prophet Muhammad through his daughter and son-in-law Fatima and Ali. The Zaydis trace their succession from Zayd ibn Ali.
The lives and the sufferings of Shi’a Imams are commemorated in story and ritual, as can be seen in the “passion plays” given on the 10th day of the month of Muharram, or the Day of Ashura, to recall the martyrdom of the Prophet’s grandson Husayn. He and members of his family were killed in a battle with the Sunni Umayyad leader Yazid I in 680 CE in modern day Iraq. The words of the Imams are also a source of law and spiritual guidance for Shi’a Muslims, in addition to the Qur’an and Sunnah of the Prophet.
In contrast to those that became Shi’as, the majority of the early community chose Abu Bakr as-Siddiq to be khalifah (caliph, or successor) 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. For them, 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 khalifahs expanded the borders of the early Muslim empire; the Umayyad Dynasty assumed the khilafah, or caliphate, after 661 CE and ruled from Damascus.
"Imam 'Ali with Hasan and Husayn Painting With Calligraphy," Persian, 19th century, Harvard Art Museums, (public domain)