Ever since the reign of the emperor (Saint) Justinian I (AD 527-565) — who attempted to drive out foreign invaders from the west and reunite the whole of the empire — the east and the west were being driven apart.
After the invasion (by the Avars and Slavs) of the Balkan peninsula and the conquest of the middle east and northern Africa by the Muslims, the east and the west were quite literally separated and it became very difficult for trade, much less religious dialogue, to continue as freely as anyone would’ve liked.
When the Franks (led by Charles I) ultimately took hold of the western half of the Roman empire in the 8th and 9th centuries, the divide between east and west became much more pronounced. Disagreements over the proper use of Icons as well as the married priesthood, the addition of the Filioque clause to the Nicene-Constantinopolitan Creed and other doctrinal disagreements only further added fuel to the fire.
A next step towards division came with the disputes over the rightful Patriarch of the see of Constantinople (Photius or Ignatius I), in which the imperial family of the east and the Pope of Rome (Nicholas) found themselves in direct conflict. Incidental to this dispute came another round of both political (quibbles over the land of Bulgaria, Illyricum and southern Italy) and theological debate (Filioque clause). However, reunion was temporarily found in the eighth General Council (the Fourth Council of Constantinople, c. AD 879-880) between the churches of the east and the see of Rome.
Due to the inclusion of the Filioque in a letter from Pope Sergius IV to Patriarch Sergius of Constantinople in 1009, from that point forward, the Roman Pope was never again included in the Diptychs of Constantinople (there is some dispute over this letter and the reasoning behind the exclusion from the Diptychs). While the Papacy and the Church of Rome “held out” for a few centuries under Frankish rule, refusing to acknowledge the addition of the Filioque to the Creed, the Roman see finally “officially” succumbed to this uncanonical and heterodox innovation in the year 1014, when the Creed was sung with the Filioque included at the coronation of emperor Henry II in Rome.
In the year 1052, a dispute arose over some Greek churches in Norman-controlled Italy that were forced to adopt Latin liturgical rites and practices. In turn, Patriarch Michael (Cerularius) of Constantinople requested that the Latin-rite churches in Constantinople adopt the Greek liturgical forms. When they refused to conform to this request, he had them shut down completely. Beyond the “usual” objections (e.g. the Filioque), Michael also strongly objected to the Latin-rite usage of “azymes” (unleavened bread) in the Eucharist.
In 1053, Patriarch Michael sent a friendly letter to Pope Leo IX, and the Pope in turn sent a group of legates to Constantinople to supposedly work towards reconciliation. Unfortunately, Cardinal Humbert (of Rome) was not entirely committed to peace, and his hot temper led to a forged bull of excommunication (directed towards Patriarch Michael, and accusing the Greeks of nonsense such as omitting the Filioque from the Creed) being slapped on the altar at Hagia Sophia, as they quickly retreated back to Rome. Michael in turn excommunicated Humbert (not the Pope), and in the year 1054, there was a definite demarcation between Rome and Constantinople, although friendly relations continued for many years.
Perhaps the greatest contributor to the schism between the east and the west was due to the Crusades, led by the so-called “Holy Roman Empire” of the west. Emperor Alexios I of Constantinople had requested assistance from Pope Urban II of Rome in the retaking of Anatolia, but this mission soon expanded to be primarily focused on the retaking of Jerusalem from the Muslim Turks.
Both Antioch and Jerusalem were retaken by the Crusaders in 1098 and 1099 respectively, but they were not returned to the Byzantine empire. Instead, the Latins installed new Latin Patriarchs (alongside the Greeks) in both places, and the population in Antioch refused to accept the new Latin Patriarch, leading to a local schism. When the Muslim leader Saladin recaptured Jerusalem in 1187, the local population of Jerusalem was left divided between both a Latin and Greek Patriarch, bringing schism down to the popular level (being no longer isolated to the Patriarchal or imperial level).
The “Great Schism” was made final when the Fourth Crusade (at the suggestion of Alexius, son of Isaac Angelus, the recently dispossessed emperor of Constantinople) embarked to the city of Constantinople in the year 1204. When attempts to restore Angelus to the throne of Byzantium failed, the crusaders sacked Constantinople and sieged the city for three brutal days, plundering countless relics and icons, destroying churches, killing peasants and leaving destruction in their wake. A Latin kingdom was established in Constantinople, and this endured until the year 1261, when the Greeks retook their city from the Latins.
The schism between the east and the west was finally complete.