Filioque

===East–West controversy===
{{Main|East–West Schism}}
Eastern opposition to the {{lang|la|Filioque}} strengthened after the 11th century East–West Schism. According to the synodal edict, a Latin anathema, in the excommunication of 1054, against the Greeks included: "{{lang|la|ut Pneumatomachi sive Theomachi, Spiritus sancti ex Filio processionem ex symbolo absciderunt}}"{{sfn|Will|1861|p=163}} ("as pneumatomachi and theomachi, they have cut from the Creed the procession of the holy Spirit from the Son").{{Whose translation|date=November 2015}} The Council of Constantinople, in a synodal edict, responded with anathemas against the Latins:"{{sfn|Will|1861|p=159|ps=: "{{lang|grc|πρὸς ἐπὶ πᾶσι δὲ τούτοις μηδὲ ἐννονειν όλως εθελοντές, ἐν οἷς τὸ πνεῦμα οὐκ ἐκ τοῦ πατρός, ἀλλὰ καὶ ἐκ τοῦ υἱοῦ φασὶν ἐκπορεύεθαι, ὅτι ούτε από εὐαγγελιστῶν τὴν φωνὴν ἔχουσι ταύτην, ούτε από οικουμενικής συνόδου τὸ βλασφήμων κέκτηνται δόγμα. Ὁ μὲν γὰρ ὁ θεὸς ήμάν φησί: "τὸ πνεῦμα τῆς ἀληθείας ὃ παρὰ τοῦ πατρὸς ἐκπορεύεται". Οἱ δὲ τῆς κοινῆς δυσσεβείας πατέρος τὸ πνεῦμα φασὶν, ὃ παρὰ τοῦ πατρὸς καὶ τοῦ υἱοῦ ἐκπορεύεται}}}} ("And besides all this, and quite unwilling to see that it is they claim that the Spirit proceeds from the Father, not [only], but also from the Son – as if they have no evidence of the evangelists of this, and if they do not have the dogma of the ecumenical council regarding this slander. For the Lord our God says, "even the Spirit of truth, which proceeds from the Father (John 15:26)". But parents say this new wickedness of the Spirit, who proceeds from the Father and the Son."{{Whose translation|date=November 2015}})

Two councils that were held to heal the break discussed the question.

The [[Second Council of Lyon]] (1274) accepted the profession of faith of Emperor [[Michael VIII Palaiologos]]: "We believe also {{angle bracket|in}} the Holy Spirit, fully, perfectly and truly God, proceeding from the Father and the Son, fully equal, of the same substance, equally almighty and equally eternal with the Father and the Son in all things."{{sfn|DH|2012|loc=n. 853}} and the Greek participants, including Patriarch [[Joseph I of Constantinople]] sang the Creed three times with the {{lang|la|Filioque}} clause. Most Byzantine Christians feeling disgust and recovering from the Latin Crusaders' conquest and betrayal, refused to accept the agreement made at Lyon with the Latins. Michael VIII was excommunicated by [[Pope Martin IV]] in November 1281,<ref>{{cite book|last=Reinert|first=Stephen W.|year=2002|chapter=Fragmentation (1204–1453)|chapter-url=https://books.google.com/books?id=aItUoYO90UwC&pg=PA258|editor-last=Mango|editor-first=Cyril|title=The Oxford History of Byzantium|publisher=Oxford University Press|isbn=978-0-19814098-6|page=258}}</ref> and later died, after which Patriarch Joseph I's successor, [[Patriarch John XI of Constantinople]], who had become convinced that the teaching of the Greek Fathers was compatible with that of the Latins, was forced to resign, and was replaced by [[Patriarch Gregory II of Constantinople]], who was strongly of the opposite opinion.{{sfn|Bulgakov|2004|p=104}}

Lyons II did not require those Christians to change the recitation of the creed in their liturgy.

Lyons II stated "that the Holy Spirit proceeds eternally from the Father and the Son, not as from two principles, but one, not from two spirations but by only one," is "the unchangeable and true doctrine of the orthodox Fathers and Doctors, both Latin and Greek."{{sfn|DH|2012|loc=n. 850}} So, it "condemn{{interp|ed}} and disapprove{{interp|d of}} those who {{interp| |orig=presume to}} deny that the Holy Spirit proceeds eternally from Father and Son or who {{interp| |orig=rashly dare to}} assert that the Holy Spirit proceeds from the Father and the Son as from two principles, not from one."{{sfn|NAOCTC|2003}}{{sfn|DH|2012|loc=n. 850}}

[[Image:Palaio.jpg|thumb|left|John VIII Palaiologos by [[Benozzo Gozzoli]]]]
Another attempt at reunion was made at the 15th century [[Council of Florence]], to which Emperor [[John VIII Palaiologos]], [[Ecumenical Patriarch Joseph II of Constantinople]], and other bishops from the East had gone in the hope of getting Western military aid against the looming [[Ottoman Empire]]. Thirteen public sessions held in [[Ferrara]] from 8 October to 13 December 1438 the {{lang|la|Filioque}} question was debated without agreement. The Greeks held that any addition whatever, even if doctrinally correct, to the Creed had been forbidden by Ephesus I, while the Latins claimed that this prohibition concerned meaning, not words.{{sfn|ODCC|2005|loc="Florence, Council of"}}

During the Council of Florence in 1439, accord continued to be elusive, until the argument prevailed among the Greeks themselves that, though the Greek and the Latin saints expressed their faith differently, they were in agreement substantially, since saints cannot err in faith; and by 8 June the Greeks accepted the Latin statement of doctrine. Joseph II died on 10 June. A statement on the {{lang|la|Filioque}} question was included in the {{lang|la|Laetentur Caeli}} decree of union, which was signed on 5 July 1439 and promulgated the next day – Mark of Ephesus was the only bishop not to sign the agreement.{{sfn|ODCC|2005|loc="Florence, Council of"}}

The Eastern Church refused to consider the agreement reached at Florence binding,{{Explain|date=November 2015|reason=How does something that is agreed to by all representative bishops, except one, change into a lack of consensus?}} since the death of Joseph II had for the moment left it without a Patriarch of Constantinople. There was strong opposition to the agreement in the East, and when in 1453, 14 years after the agreement, the promised military aid from the West still had not arrived and [[Fall of Constantinople|Constantinople fell]] to the Turks, neither Eastern Christians nor their new rulers wished union between them and the West.