The Great Schism of 1054 was the formal split between the Catholic Church in the West (centered in Rome) and the Eastern Orthodox Churches (centered in Constantinople). While tensions had been building for centuries due to theological, political, cultural, and linguistic differences, the schism was precipitated by mutual excommunications between Pope Leo IX's legate Cardinal Humbert and Patriarch Michael Cerularius of Constantinople. Key issues included: (1) The Filioque controversy — the Western Church added 'and the Son' (Filioque) to the Nicene Creed regarding the procession of the Holy Spirit, which the East rejected. (2) Papal authority — the East acknowledged the Pope as 'first among equals' but rejected his claim to universal jurisdiction over all Christians. (3) Liturgical differences — leavened vs. unleavened bread, clerical celibacy requirements, and other practices. (4) Political rivalries between Rome and Constantinople. Although excommunications were mutually lifted in 1965 by Pope Paul VI and Patriarch Athenagoras, full communion has not been restored. Ecumenical dialogue continues, and Catholics and Orthodox share much in common, including valid sacraments and apostolic succession.