This designates the event that happened in A.D. 1054 to separate Christendom into two parts; the eastern churches under the patriarch at Constantinople and the western churches under the pope at Rome. Already the two churches were very different, both culturally and theologically. They had already developed their separate ways. Early in the eighth and ninth centuries there was discussion and writing in the West about the continuing iconoclastic problem in the Eastern Church. This iconoclastic dispute lasted for 120 years.
Political antagonism between the Eastern and the Carolingian emperors also led to an attack by the theologians in the West on the practices and beliefs of the Eastern church. These controversial works on the "errors of the Greeks" proliferated during the ninth century as a result of the "Photian Schism", when the Patriarch of Constantinople Ignatius was deposed by the Eastern Emperor in A.D. 858. The deposed Patriarch Ignatius appealed to Pope Nicholas I (A.D. 858-867), as did his replacement, Photius (A.D. c.820-c.895). Pope Nicholas sent legates to Constantinope, who joined in approving Photius. The Pope repudiated their action, and, in A.D. 863, they declared Photius deposed. At a synod in Rome in A.D. 863 Nicholas condemned Photius and declared Ignatius Patriarch. And when Pope Nicholas ordered the restoration of Ignatius as Patriarch, the relations between Constantinople and Rome worsened. Photius now accused the Western Church of heresy for adding the filique clause to the creed, fasting on Saturdays, using milk, butter and cheese in Lent, demanding priestly celibacy, and confining confirmation to the bishops. At synod under his leadership in Constantinople in A.D. 867, the Pope was condemned. Thus Pope Nicholas failed in his attempt to exercise his authority over the Eastern churches. Thus the ill feelings between East and West increased, but not enough to lead to a complete separation of the churches which finally happened in A.D. 1054.
Latin theologians also criticized the Eastern Church for its different method for deciding the date of Easter, the difference in clergymen's tonsure-style, and over the celibacy of the clergy. (The Eastern church allowed clergy to marry, but required monks to be celibate.) The major theological controversy involved the filioque question. Did the Holy Spirit descend "from the Father through the Son" or "from the Father and the Son"? From the time of Photius, Eastern theologians bitterly attacked the Western church on this issue, declaring that the Western position of "and the Son" (filioque) was a late addition to the Nicene Creed (as it indeed was). This issue further alienated the Eastern and Western churches. The Greek-speaking East and the Latin West were also divided in language. "East and West could not understand each other because they could not understand each other!"
The Great Schism that occurred in A.D. 1054 marked the end of the separation of the Latin speaking Roman church in the West from the Greek speaking church in the East. They had been growing further apart for centuries economically, politically, and culturally, but at the end the split took place for theological and doctrinal issues. One of these was the matter of papal claims. The pope was claiming absolute power in both East as well as in the West. The Greeks were willing to accord honor to the pope, but not universal supremacy. They believed that matters of faith were to be decided by a council of all the bishops of the church, not just by papal authority. The other doctrinal issue was the Filioque. Originally the Nicene-Constantinopolian Creed had said, "I believe ... in the Holy Spirit, the Lord, the Giver of Life, who proceeds from the Father, who with the Father and the Son together is worshipped and together glorified." The West inserted a phrase, so that the creed now read, "who proceeds from the Father and the Son". The Greeks objected to this change because they believed that the ecumenical councils forbade any changes in the creed, and if a change was to be made, only another ecumenical council could make it. The Greeks also believed that the change was doctrinally wrong because it destroyed the balance between the three persons of the Trinity and could lead to an incorrect doctrine of the Spirit and of the Church. And besides the major differences there were minor differences between the Greeks and the Latins, as we saw above, such as priestly celibacy in the West (the Greeks allowed married clergy), different rules of fasting, and the use of leavened bread in the Eucharist in the East (the Latins used unleavened bread).
In A.D. 1054 the final split came about over an apparently minor matter. Michael Cerularius, patriarch of Constantinople from A.D. 1043 to 1059, in conjunction with Leo, the metropolitan of Bulgaria, closed churches of the Latin rite in their regions, and attacked the Latin Church in a letter written by the latter pressing Photius' old charges that accused the Latin churches of heresy by adding the filique clause to the creed, fasting on Saturdays, using milk, butter and cheese in Lent, demanding priestly celibacy, and confining confirmation to the bishops. He also added a condemnation of the church in the West for using unleavened bread in the Eucharist. Such use had been a growing practice in the West since the ninth century. Pope Leo IX sent Cardinal Humbert and two other legates to the East to resolve and end the dispute. The difference of opinion widened as the discussion went on. Things went from bad to worst. On July 16, A.D. 1054, the Roman legates finally put a decree of excommunication of the patriarch and his followers on the high altar of the catheral church of Saint Sophia. This act has been usually considered as the formal separation of the Latin and Greek Churches. The patriarch was not to be outdone, and thereupon in synod he anathematized the pope of Rome and his followers. This event is the first great schism in Christianity that broke the unity of the church. From that time on the Roman Catholic church and the Greek Orthodox church went their separate ways. This mutual excommunication was not removed until Dec. 7, 1965, when it was removed by both Pope Paul VI and patriarch Athenagoras.