Imamate

When Muhammad died in Medina in 632 CE without naming an unambiguous successor, the community's leading men gathered at a hall called Saqifat Bani Sa'ida and chose Abu Bakr, an early companion, as caliph, while Ali ibn Abi Talib and Muhammad's household were still occupied with his burial. Sunni tradition remembers this as a legitimate act of consultation among the believers, the model for the three caliphates that followed under Abu Bakr, Umar and Uthman before Ali himself finally became the fourth caliph. Shia tradition remembers it as a usurpation. In the Shia reading, Muhammad had already designated Ali at Ghadir Khumm, a watering place on the road back from his final pilgrimage, where he is reported to have told the assembled pilgrims that whoever held him as master should hold Ali as master too.

Sunni Islam accepts that the event occurred but reads the words as an expression of closeness and affection, not a formal transfer of religious authority. Ali did eventually become caliph in 656 CE, only to be assassinated five years later by a Kharijite in the mosque at Kufa, and the dispute did not end with him. His son Husayn was killed at Karbala in 680 CE resisting the Umayyad caliph Yazid, an event that gave Shia Islam its central narrative of righteous suffering under unjust worldly power, marked every year at Ashura and still central to how Shia Muslims understand legitimate authority.

The branches of Islam that trace authority to Ali diverge sharply among themselves on how it continued. Twelver Shia Islam, the largest Shia branch, holds that the imamate passed through a fixed line of twelve men, each designated by his predecessor and each held to be divinely protected from error, ending with Muhammad al-Mahdi, believed to have gone into hiding as a child in 874 CE and to remain alive in occultation until he returns to establish justice. Ismaili Shia Islam split from the Twelver line in the eighth century over which son should have succeeded the sixth Imam, Ja'far al-Sadiq, and its main surviving branch, the Nizaris, holds that the imamate has continued in an unbroken living succession ever since, currently held by the Aga Khan, rather than passing into hiding.

Zaydi Shia Islam split earlier still and takes a more activist view, holding that any sufficiently learned descendant of Ali and Fatima who publicly claims the office and can enforce it by rising against injustice may become Imam, which produced a long line of separate Zaydi imamates in Yemen rather than one fixed lineage. Sunni Islam rejects the underlying premise that leadership was ever divinely reserved to Ali's line, holding instead that the community selects and may remove its own leader, formalized in its classical political theory of the caliphate. Ibadi Islam, descended from the moderate wing of the earliest Kharijite movement, holds a fourth position again: that the imamate belongs to whichever believer is most qualified by piety and knowledge regardless of tribe or descent, and that an imam who turns unjust may rightly be deposed by the community that raised him up.