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Patriarch of Constantinople who played a key role in the events leading to the Great Schism by challenging the authority of the Western Church, eventually leading to mutual excommunication.

Pope of the Roman Church who sent a delegation to Constantinople that ultimately excommunicated Michael Cerularius, contributing to the Great Schism.

A Latin term meaning 'and the Son', which was added to the Nicene Creed in the Western Church, creating theological conflict with the Eastern Church and contributing to the Great Schism.

A delegate sent by Pope Leo IX to Constantinople, who laid a bull of excommunication on the altar of Hagia Sophia, exacerbating the Great Schism.

The division in Christian unity that occurred in 1054, caused by theological and political differences, and disputes over ecclesiastical authority between the Eastern (Orthodox) and Western (Roman Catholic) Churches.