Arsenite Schism

The Arsenite Schism was a conflict dividing the Byzantine Church between 1265 and 1310. The schism began when a church synod deposed the Ecumenical Patriarch of Constantinople Arsenios Autoreianos on the initiative of Emperor Michael VIII Palaiologos.

Background
The Council of Chalcedon divided the ecclesiastic administration of Christianity between the five most prominent bishops of the Roman Empire. They were the bishops of Rome, Constantinople, Alexandria, Antioch and Jerusalem, each of them styled as patriarchs. Under this model of church administration, known as pentarchy, the four eastern patriarchs acknowledged the honorary primacy of the bishops of Rome (or popes), but maintained that the entire church was under the collective leadership of the five patriarchs, and denied the right of any patriarchal see to intervene in the affairs of an other patriarchate.

During the following centuries, the western Church adopted practices, such as the use of unleavened bread in the Eucharist and fasting on Saturday, that remained alien to eastern Christians. The unilateral modification of the Nicene Creed regarding the Holy Spirit—one of the three divine persons of God in Christianity—in the west caused further conflicts. The eastern Christians maintained that the Holy Spirit proceeded only from God the Father, whereas western Christians began to attribute a role to God the Son in the process. The westen concept known as filioque ('and from the Son') was officially introduced in Rome early in the 11th century, demonstrating the papacy's claim to supreme authority. The conflict culminated in the East–West Schism of 1054. This year papal legates excommunicated the Ecumenical Patriarch of Constantinople, Michael I Cerularius (d. 1059) for his resistance to papal demands relating the use of unleavened bread; in response, Cerularius excommunicated the papal legates.

During the Fourth Crusade in 1204–1205, a coalition of western knights and Venetian merchants captured Constantinople, and seized large chunks of the Byzantine Empire. The westerners transformed the conquered territories into the Latin Empire of Constantinople, but Byzantine resistance survived in the unconquered lands. The Byzantine aristocrat Theodore I Laskaris ((r. 1205 – 1221)) took control of the Byzantine lands in Asia Minor, transforming them in a Byzantine successor state, the Empire of Nicaea. With his active support, the Eastern Orthodox clergy of Constantinople elected Michael IV Autoreianos (d. 1212) as the new Ecumenical Patriarch in 1208. The new Patriarch established his seat in Nicaea, and crowned Theodore emperor.