Churches in the East had become alienated from the Church in Rome beginning in the fifth century because of theological disagreements about Christ and his works, and growing more serious in 1054. But it wasn't until the sacking of Constantinople in 1203 that the schism hardened. (The Pope had sent the Fourth Crusade to recapture the city, but the crusaders aligned themselves instead with a Byzantine prince who wanted the city for himself.)

Two councils, Lyon in 1274 and Florence in 1439, attempted unsuccessfully to breach the fracture. In 1965, the Second Vatican Council began to clear the way for reconciliation between the churches.