The Great Schism
East and West part company!
In the 11th century what was recognised as the "Great Schism" took place between Rome and Constantinople, which led to separation between the Church of the West, the Roman Catholic Church, and the Eastern Byzantine Churches, now known as the Orthodox Church. There were doctrinal issues like the "filioque" clause and the authority of the Roman Pope involved in the split, but these were greatly exacerbated by political factors of both Church and state, and by cultural and linguistic differences between Latins and Greeks. The first formal split occurred in the year 1054 - the "Great Schism".
Prior to 1054, the Eastern and Western halves of the Church had frequently been in conflict, particularly over the "filioque" clause. The first ecumenical council, that of Nicaea (325), ended its Creed with the words "and in the Holy Spirit". The second, that of Constantinople in 381, spoke of the Holy Spirit as "proceeding from the Father". This last phrase is based on John 15:26.
The third ecumenical council, held at Ephesus in 431, which quoted the creed in its 325 form, forbade setting up a different creed as a rival to that of the first ecumenical council. This unilateral decree did not go down well in Constantinople! The creed of the second ecumenical council was adopted liturgically in the East and later a Latin variant was adopted in the West. The form of this creed that the West adopted' however, had two additions: "God from God" (Deum de Deo) and (…proceeds from the Father) "and the Son" (Filioque). It was this Latin variation that sparked the fire of conflict that was eventually to lead to what is known as the "Great Schism" of 1054.
Although already formally separated since 1054, the Roman and Orthodox churches continued, grudgingly and intermittently, to recognise the authority of the other for another 400 years!
However, the capture and sacking of Constantinople by the Fourth Crusade and establishment of the Latin Empire as a seeming attempt to supplant the Orthodox Byzantine Empire in 1204 is viewed with some rancour even to the present day. Reunion was attempted at the 1274 Second Council of Lyon, but conflict continued to flare up for many years. The 1439 Council of Florence did briefly reestablish communion between East and West, which lasted until after the fall of Constantinople to the Ottoman Empire in 1453. In both cases, however, the councils were rejected by the Orthodox people as a whole and became very politically difficult after Constantinople came under Ottoman rule. The final break with Rome occurred in 1453.
In 2004, Pope John Paul II extended a formal apology for the sacking of Constantinople in 1204, which, incidentally, was also strongly condemned by the Pope at the time (Innocent III). The apology was formally accepted by Patriarch Bartholomew of Constantinople. Many things that were stolen during this time - holy relics, riches, and many other items - were not returned and are still held in various Western European cities, particularly Venice.
Despite the apology being offered and accepted; only some local Eastern Churches have re-established union with Rome.