On 16 July 1054, the emissaries of Pope Leo IX led by Cardinal Humbert of Mourmoutiers entered the Basilica of the Hagia Sophia in Constantinople during the celebration of the Divine Liturgy and placed a bull of excommunication against the Patriarch of Constantinople Michael Cerularius atop the altar. The priests of Constantinople chased after the legates, begging them to take back the bull of excommunication in vain. Thus initiating the present split between the Catholic and Orthodox Churches was begun.
The immediate source of the conflict was the practice of the Normans in southern Italy of forcing the Greeks to adopt Latin practices. In response Patriarch Cerularius had forced Latin churches in Constantinople to adopt Greek practices. Cardinal Humbert was sent to Constantinople in order to deal with this conflict.
While Humbert had been sent to to Constantinople to deal with the conflict over ritual differences between East and West – such as use of unleavened bread in the West in the Eucharist; the poor treatment of the legates was what truly led him to issue this bull of excommunication accusing the Patriarch of simony, rebaptizing Latins, allowing priests to marry, baptizing women in labor, abandoning the Mosaic law, refusing communion to men who had shaven their beards and omitting the filioque clause in the Creed.
In many of these accusations Humbert was incorrect and and ignorant of Eastern practice; moreover, Pope Leo had died some time prior to this event, therefore Humbert also did not have the proper authority to issue this excommunication. His status as papal legate came to an end with the death of the pope who appointed him.
After the legates left Constantinople, the bull of excommunication was burnt and the Church in Constantinople excommunicated Humbert and the other papal legates but did not excommunicate the pope.
However, it was Humbert’s version of events which would color the West’s image of the East and keep the schism alive. The West, believing that it held primacy over the entire Church, wanted the East to acknowledge its errors, the East did not believe that any errors had been made and refused to recognize the West’s understanding of papal authority.
While most historians place this date as the beginnings of the split between East and West, it would be the Crusader Sack of Constantinople in 1204 which would make this split permanent.
On 7 December 1965, Pope Paul VI and Ecumenical Patriarch Athenagoras lifted these mutual excommunications.