The Great Eastern Schism is one of the darkest moments in Church history. Just like subsequent schisms, it was based on politics under the guise of Church doctrine. The Church in the East, based in Constantinople, started to become a state church of the emperor, whereas the Church in the West, based in Rome, aligned its strengths with the kings of France and emperor of the Holy Roman Empire.
The division came about in a matter of Church policy and governance. Ignatios was the Patriarch of Constantinople from 847–858, and was the son of the Byzantine Emperor Michael. He had arrived just after the second wave of iconoclasm (heresy denying the use of icons, statues, or any images of Jesus and the saints) which lasted from 814–842. In his zeal to restore icons and orthodoxy to the Eastern Church within the Eastern Byzantine Empire, he stepped on some toes, including those of the Archbishop of Syracusa who appealed to the bishop of Rome, Pope Leo IV, for help. This caused tension between the East and West of the Church. Ignatios was later deposed in 858 and replaced by Photios, who was installed by Emperor Michael III (not the same Emperor Michael who was Ignatios’ father). Ignatios then appealed to Pope Nicholas I just as the Archbishop of Syracusa had done to Pope Leo IV. Photios was deposed in 867 when the emperor was murdered, and Ignatios reclaimed the office of patriarch of Constantinople from 867–877. When he died, Photios returned as patriarch from 877–886. The political intrigue back and forth between the Byzantine Emperor and the pope in Rome only continued to escalate as the patriarchs of Constantinople fell under the tight grip and control of the Eastern Emperor. The pope, on the other hand, had no secular rival, as the barbarians had defeated the Roman emperors back in 476.
Relations between the pope in Rome and the patriarch of Constantinople were at an all-time low in the eleventh century. Patriarch Michael Cerularius considered the Latin Catholics to be heretics since they used unleavened bread in their Mass whereas Eastern Christian custom is to use only leavened bread. Sicily had been under Byzantine control for centuries until the Normans conquered and established Latin (Western) Church customs and clergy. Pope Leo IX in 1048 sought to reestablish Latin control over the Italian peninsula and to encourage—or, if necessary, coerce—the Byzantines to return back to the East, at least to Constantinople, and leave the Western church to the control of the bishop of Rome.
Patriarch Cerularius retaliated in 1052 by closing all Latin churches in Constantinople in response to the Greek Byzantine churches in Sicily being “Romanized” as he called it. Pope Leo sent Cardinal Humbert as his legate to sort things out. Meanwhile, Cerularius began to stir up antagonism against the Latin Church by complaining that the Roman Pope inserted the “filioque” into the Nicene Creed (325 AD) without proper authority. While the pope has such authority as Supreme Head of the Church and Successor of Saint Peter, in reality it was the Catholic Church in sixth century Spain that inserted the filioque into the Nicene Creed, and it became more prevalent and popular in the West over time.
To this day, the Eastern Orthodox Church and the Byzantine Catholic Church omit the words “and the Son” (filioque in Latin) from the part of the Creed which reads that the Holy Spirit “proceeds from the Father.” The West (Latin) Church retains this phrase, however, and professes every Sunday and holy day that the Holy Spirit “proceeds from the Father and the Son.” A theological controversy of minor proportions, the filioque became the smokescreen by which both Cerularius and Humbert put a final wedge between East and West.
Cardinal Humbert, legate of Pope Leo IX, entered Hagia Sophia (1054 AD) in Constantinople and left the papal bull of excommunication of Patriarch Cerularius on the altar. And thus the Schism between East and West formally began.