The Greek East and the Latin West grew apart over centuries before any formal break. Their differences were of doctrine, custom, language and politics: among them the filioque added to the Creed in the West, whether leavened or unleavened bread should be used in the Eucharist, the pope's claim to universal jurisdiction over all the churches, and the place of the See of Constantinople among the ancient patriarchates. 1054 is the conventional date; some scholars have proposed others, among them 1009, 1204, 1277 and 1484.
In 1053 Patriarch Michael Cerularius of Constantinople ordered the closure of the Latin churches in Constantinople. In 1054 Pope Leo IX sent legates to the city, led by Cardinal Humbert. When Cerularius would not accept their demand that he recognize the pope's claim to be head of all the churches, Humbert excommunicated him, and Cerularius in turn excommunicated the legates. The validity of the legates' act is doubtful, because Pope Leo had died, and Cerularius' excommunication applied only to the legates personally. Still, the Church split along doctrinal, theological, linguistic, political and geographical lines.
Later events deepened it. In 1204 the armies of the Fourth Crusade sacked and pillaged Constantinople and set Latin patriarchs in place of the Greek ones. Attempts at reunion in the centuries that followed did not bear fruit, and each side has at times held the other responsible for the division.
In 1965 Pope Paul VI and Ecumenical Patriarch Athenagoras I lifted the excommunications of 1054. It was a gesture of goodwill, not a reunion, and the Orthodox and Catholic Churches remain out of communion. Their theological dialogue continues through a joint international commission that first met in 1980.