The Great Schism
The East–West Schism, also called the Great Schism and the Schism of 1054, was the break of communion between what are now the Eastern Orthodox and Catholic churches, which has lasted since the 11th century.
The ecclesiastical differences and theological disputes between the Greek East and Latin West pre-existed the formal rupture that occurred in 1054. Prominent among these were the issues of the source of the Holy Spirit, whether leavened or unleavened bread should be used in the Eucharist, the Bishop of Rome's claim to universal jurisdiction, and the place of the See of Constantinople in relation to the Pentarchy.
Tensions Between East and West
By the turn of the millennium, the Eastern and Western Roman Empires had been gradually separating along religious fault lines for centuries, beginning with Emperor Leo III's pioneering of the Byzantine Iconoclasm in 730 CE, in which he declared the worship of religious images to be heretical. The Western church remained firmly in support of the use of religious images. Leo tried to use military force to compel Pope Gregory III, but he failed, and the pope condemned Leo's actions. In response Leo confiscated papal estates and placed them under the governance of Constantinople.
Therefore, the Iconoclasm widened the growing divergence and tension between East and West, though the church was still unified at this time. It also decisively ended the so-called Byzantine Papacy, under which, since the reign of Justinian I a century before, the popes in Rome had been nominated or confirmed by the Emperor in Constantinople. Now the deference of the Western church to Constantinople had dissolved, and Rome would maintain a consistently iconodule position (supports or is in favor of religious images or icons and their veneration).
A New Emperor in the West
Regent Irene convened the Second Council of Nicaea in 787 CE, which temporarily restored image worship, in an attempt to soothe the strained relations between Constantinople and Rome—but it was too late. After Charlemagne, the king of the Franks, saved Rome from a Lombard attack, Pope Leo III (not to be confused with the Byzantine Leo III) declared him the new Roman Emperor in 800 CE since a woman (Irene) could not be emperor. It was also a message that the popes were now loyal to the Franks, who could protect them, instead of the Byzantines, who had only caused trouble. To the Byzantines, this was an outrage, attacking their claim to be the true successors of Rome.
From this point on, the Frankish Empire is usually known as the Holy Roman Empire. With two Roman Empires, the Byzantines and the Franks, the authority of the Byzantine Empire was weakened. In the West they were no longer called "Romans," but "Greeks" (and eventually "Byzantines"). The Byzantines, however, continued to consider themselves Romans, and looked to the patriarch of Constantinople, not the pope, as the most important religious figure of the church.
Crisis and Permanent Schism
The differences in practice and worship between the Church of Rome in the West and the Church of Constantinople in the East only increased over time.
In 1053, the first step was taken in the process which led to formal schism: the Ecumenical Patriarch of Constantinople Michael I Cerularius ordered the closure of all Latin churches in Constantinople, in response to the Greek churches in southern Italy having been forced to either close or conform to Latin practices. According to the historian J. B. Bury, Cerularius' purpose in closing the Latin churches was "to cut short any attempt at conciliation."
Finally, in 1054 CE, relations between the Eastern and Western traditions within the Christian Church reached a terminal crisis. The papal legate sent by Leo IX traveled to Constantinople for purposes that included refusing to Cerularius the title of "Ecumenical Patriarch" and insisting that he recognize the Pope's claim to be the head of all the churches. The main purpose of the papal legation was to seek help from the Byzantine Emperor in view of the Norman conquest of southern Italy and to deal with recent attacks by Leo of Ohrid against the use of unleavened bread and other Western customs, attacks that had the support of Cerularius. Historian Axel Bayer contends that the legation was sent in response to two letters, one from the Emperor seeking assistance in arranging a common military campaign by the eastern and western empires against the Normans, and the other from Cerularius. On the refusal of Cerularius to accept the demand, the leader of the legation, Cardinal Humbert of Silva Candida, excommunicated him, and in return Cerularius excommunicated Humbert and the other legates. This was only the first act in a centuries-long process that eventually became a complete schism.
The gradual separation of the last several centuries culminated in a formal declaration of institutional separation between East, into the Orthodox Church (now Eastern Orthodox Church), and West, into the Catholic Church (now Roman Catholic Church). This was known as the East-West Schism.
The East-West Schism
The religious distribution after the East-West Schism between the churches of the Byzantine Empire and the Holy Roman Empire in 1054 CE.
The Church split along doctrinal, theological, linguistic, political, and geographical lines, and the fundamental breach has never been healed, with each side sometimes accusing the other of having fallen into heresy and of having initiated the division. Conflicts over the next several centuries (such as the Crusades, the Massacre of the Latins in 1182 CE, the West's retaliation in the Sacking of Thessalonica in 1185 CE, the capture and sack of Constantinople in 1204 CE, and the imposition of Latin patriarchs) would only make reconciliation more difficult.