The council opened at Basel in 1431 under a mandate from Pope Martin V, who died before its first session, and continued under his successor Eugene IV. When its majority asserted that a general council held authority over the pope, a position known as conciliarism, and moved to negotiate independently with the Hussites of Bohemia, Eugene IV sought to transfer the assembly to a city under closer papal control. Using the arrival of a Byzantine delegation seeking Western aid against the Ottoman Turks as his occasion, he moved the council first to Ferrara in 1438 and then, when plague broke out and Florence offered financial support through Cosimo de' Medici, to Florence in January 1439.
There, Emperor John VIII Palaiologos, Patriarch Joseph II of Constantinople and a large delegation of Greek bishops, including Bessarion of Nicaea and Mark of Ephesus, debated the Latin and Greek churches' long disputed points: the Filioque clause added to the Latin creed, the nature of purgatory, and whether leavened or unleavened bread was proper for the Eucharist. On 6 July 1439 the bull Laetentur Caeli proclaimed a formula of union on essentially Latin terms, accepting the Filioque under an accommodating reading and affirming papal primacy. Mark of Ephesus alone among the Greek bishops present refused to sign it.
The Filioque dispute turned on a single clause added to the Latin text of the Nicene Creed centuries earlier, asserting that the Holy Spirit proceeds from the Father and the Son rather than from the Father alone, as the Greek text still read; the two churches had argued over the addition, and the authority to make it, since at least the ninth century, and Florence's formula tried to accommodate both readings as compatible rather than choosing between them.