East–West Schism

16 July 1054, on the high altar

Humbert of Silva Candida and his two fellow Roman legates entered Hagia Sophia in Constantinople on 16 July 1054 and deposited a charter of excommunication on its high altar.

Accounts differ over whether the Divine Liturgy was already under way or still being prepared, so "during the service" cannot be asserted flatly.

Named people, not two churches

The sentence targeted Patriarch Michael Keroularios and two other persons, or their adherents. It was not a blanket excommunication of every Eastern Christian, or of the Byzantine Church.

Within days Michael convened a Constantinopolitan synod that condemned and anathematized Humbert and his companions — again specific men, not every Western Christian.

On whose authority

Pope Leo IX, who had commissioned the mission, had died on 19 April, about three months before the charter reached the altar. Whether that voided the legates' mandate is a disputed canonical question; at minimum, their authority was seriously contested.

Negotiations had already collapsed over papal authority and over Latin and Greek practices, Eucharistic bread above all, after Keroularios refused to recognize or negotiate with the mission.

Modern historians generally reject 16 July 1054 as the exact day two fully formed denominations permanently split; intercommunion and negotiation continued afterward.