Three Roman legates break relations between Western and Eastern Christian churches through the act of placing a papal bull of excommunication on the altar of Hagia Sophia during Saturday afternoon divine liturgy. Historians frequently describe the event as the formal start of the East–West Schism.

A papal legate or apostolic legate is a personal representative of the Pope to foreign nations, to some other part of the Catholic Church, or to representatives of a state or monarchy. A legate is empowered in matters of Catholic faith and for the settlement of ecclesiastical matters.

Western Christianity is one of two subdivisions of Christianity. Western Christianity is composed of the Latin Church and Western Protestantism, together with their offshoots such as the Old Catholic Church, Independent Catholicism and Restorationism.

Eastern Christianity comprises Christian traditions and church families that originally developed during classical and late antiquity in the Eastern Mediterranean region or locations further east, south or north. The term does not describe a single communion or religious denomination. Eastern Christianity is a category distinguished from Western Christianity, which is composed of those Christian traditions and churches that originally developed further west.

A papal bull is a type of public decree, letters patent, or charter issued by the pope of the Catholic Church. It is named after the leaden seal (bulla) traditionally appended to authenticate it.

Excommunication is an institutional act of religious censure used to deprive, suspend, or limit membership in a religious community or to restrict certain rights within it, in particular those of being in communion with other members of the congregation, and of receiving the sacraments.