East–West Schism
Papal supremacy and Pentarchy

The primary causes of the schism were disputes over conflicting claims of jurisdiction, in particular over papal authority — Pope Leo IX claimed he held authority over the four Eastern patriarchs —and over the insertion of the Filioque clause into the Nicene Creed by the Western patriarch in 1014. Eastern Orthodox today state that Council of Chalcedon canon 28 explicitly proclaimed the equality of the Bishops of Rome and Constantinople and that it established the highest court of ecclesiastical appeal in Constantinople. Council of Ephesus canon 7 declared:

It is unlawful for any man to bring forward, or to write, or to compose a different (ἑτέραν) Faith as a rival to that established by the holy Fathers assembled with the Holy Ghost in Nicæa. But those who shall dare to compose a different faith, or to introduce or offer it to persons desiring to turn to the acknowledgment of the truth, whether from Heathenism or from Judaism, or from any heresy whatsoever, shall be deposed, if they be bishops or clergymen; bishops from the episcopate and clergymen from the clergy; and if they be laymen, they shall be anathematized

Eastern Orthodox today state that this canon of the Council of Ephesus explicitly prohibited modification of the Nicene Creed drawn up by the first Ecumenical Council in 325, the wording of which, it is claimed, but not the substance, had been modified by the second Ecumenical Council, making additions such as "who proceeds from the Father".

Eastern Orthodox argue that the First Council of Ephesus canon 7 explicitly prohibited modification of the Nicene Creed by any man (not by ecumenical church council) drawn up by the first Ecumenical Council in 325. In reality, the Council made no exception for an ecumenical council or any other body of bishops, and the Greeks participating in the Council of Florence emphatically denied that even an ecumenical council had the power to add anything to the creed.

The creed quoted in the Acts of the Council of Ephesus of 431 (the third ecumenical council) is that of the first ecumenical council without the modifications that the second ecumenical council, held in Constantinople in 381, is understood to have made to it, such as the addition of "who proceeds from the Father". Eastern Orthodox theologians state this change of the wording of the churches' original creed was done to address various teachings outside of the church—specifically, that of Macedonius I of Constantinople, which the council claimed was a distortion of the church's teaching on the Holy Spirit. This was not a change of the orthodoxy of the churches' original creed.

Thus the word ἑτέραν in the seventh canon of the later Council of Ephesus is understood as meaning "different" or "contradictory" and not "another" in the sense of mere explanatory additions to the already existing creed. Some scholars hold that the additions attributed to the First Council of Constantinople were adopted only with the 451 Council of Chalcedon, 20 years after that of Ephesus, and even that the Council of Ephesus, in which Alexandrian influence was dominant, was by this canon excluding the Constantinopolitan Creed, which eventually annexed the name and fame of the creed adopted at Nicaea.
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