[2] During the reign of John V Palaiologos in the preceding century, the Byzantine Emperor had issued pleas to the West for aid in exchange for a union of the Roman Catholic and Greek Orthodox churches; the Papacy had been unmoved by these appeals,[4] as had been King Louis I of Hungary.

He consulted with Neoplatonist philosopher Gemistus Pletho, who advised him that the Roman Catholic and Eastern Orthodox delegations should have equal voting power at the Council;[6] nonetheless, the Emperor was under far more pressure to bring about a union than was the Pope.

The chief difference revolved around the insertion of the word Filioque (English: and the Son) into the Latin version of the Nicene Creed by the Roman Catholic Church, which Orthodox bishops had refused to accept.

Sublatus est enim de medio paries qui occidentalem orientalemque dividebat ecclesiam et pax atque concordia rediit, illo angulari lapide Christo qui fecit utraque unum, vinculo fortissimo caritatis et pacis utrumque iungente parietem et perpetue unitatis federe copulante ac continente; postque longam meroris nebulam et dissidii diuturni atram ingratamque caliginem, serenum omnibus unionis optate iubar illuxit.

[10] Initially, Eastern Orthodox Patriarch Joseph II of Constantinople was in attendance, but when he died before the council ended, Emperor John VIII largely took Church matters into his own hands.

[11] In return, Eugene pledged to provide military assistance for the defence of Constantinople and to encourage the King of Germany Albrecht II to war against the Ottomans.

[13] It has been suggested that Eugenius IV insisted on this because his primacy was at the time being threatened by a rival Antipope, Felix V, and the Conciliar Movement at the Council of Basel.

[14] On the subject of the Filioque, it took a similar tone, emphasizing the commonalities between the theologies of the East and West but clearly siding with the Roman Catholic position without even mentioning Eastern Orthodox objections.

Finally, the bull defined the order of primacy among the patriarchs of the pentarchy as being Rome, Constantinople, Alexandria, Antioch, and lastly Jerusalem.

[1] The lone dissenting voice against the bull was that of Mark of Ephesus, delegate for the Patriarch of Alexandria, who refused to compromise on either the Filioque or Purgatory and held that Rome continued in heresy and schism.

[11] Reportedly, upon seeing that Mark's signature was missing, Eugene IV responded, “And so we have accomplished nothing.”[11] Nonetheless the union was to proceed, and representatives from the Vatican would be sent to Constantinople to see how it was being carried out.

After a long haze of grief and a dark and unlovely gloom of long-enduring strife, the radiance of hoped-for union has illuminated all.

We also define that the holy apostolic see and the Roman pontiff holds the primacy over the whole world and the Roman pontiff is the successor of blessed Peter prince of the apostles, and that he is the true vicar of Christ, the head of the whole church and the father and teacher of all Christians, and to him was committed in blessed Peter the full power of tending, ruling and governing the whole church, as is contained also in the acts of ecumenical councils and in the sacred canons.

He signed an agreement with the Armenians on 22 November 1439,[15] and with a part of the Jacobites of Syria in 1443, and in 1445 he received some of the Nestorians and the Cypriot portion of the Maronites.

[12] Władysław nonetheless undertook the crusade with Hungarian troops and was killed in the Battle of Varna within a year, ending the attempt.

[7][8][11] The newly Eastern Catholic bishops, in opposition to Mark, attested his resistance: “Having returned to Constantinople, Ephesus disturbed and confused the Eastern Church by his writings and addresses directed against the decrees of the Council of Florence.”[11] Opinion among the bishops in Russia, contrary to those in Constantinople, remained with Mark, and by 1443 most Russian patriarchs repudiated the Council of Florence and the union of the churches.

[11] Thus Isidore of Kiev was arrested at the command of Vasily II upon his return to Moscow and convicted of apostasy, after which he was imprisoned; he then escaped and fled to Rome to become a cardinal.

The new Ottoman rulers wanted to prevent the conquered Byzantines from appealing to the West, so Sultan Mehmed the Conqueror appointed the anti-union Gennadius Scholarius as Ecumenical Patriarch of Constantinople; he immediately renounced the Filioque and the Great Schism was renewed.