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The Council of Basel, Ferrara and Florence

Effected a short-lived union with the Greek Church at Florence.

1431–1439 Basel · Ferrara · Florence
Under
Pope Eugene IV, Sigismund being Emperor
What it did
met first at Basel for the religious pacification of Bohemia; after quarrels with the pope it was transferred to Ferrara (1438), then Florence (1439), where a short-lived union with the Greek Church was effected
How far it is ecumenical
“only ecumenical till the end of the twenty-fifth session”, and of its decrees Eugene IV approved only those on the extirpation of heresy, the peace of Christendom and the reform of the Church

These facts as given by the Catholic Encyclopedia (1913), "Councils" PD

Scholastic

c. 1100 – 1500
1431
Event
The Council of Basel, Ferrara and Florence

At Florence a union with the Greek Church is signed. It does not last.

1439
A.D.
“In the name of the Holy Trinity, of the Father and of the Son and of the Holy Spirit, with the approval of this sacred universal Council of Florence we define that this truth of faith be believed and accepted by all Christians, and that all so profess it: that the Holy Spirit is eternally from the Father and the Son, and has his essence and his subsistent being from the Father together with the Son, and proceeds from both eternally as from one principle and by a single spiration; declaring that what the holy Doctors and Fathers say — that the Holy Spirit proceeds from the Father through the Son — tends to this understanding, that by it is signified that the Son too is, according to the Greeks the cause, and according to the Latins the principle, of the subsistence of the Holy Spirit, as is the Father. And since all that is the Father's, the Father himself in begetting gave to his only-begotten Son, save being Father, this very thing — that the Holy Spirit proceeds from the Son — the Son himself has eternally from the Father, by whom he was likewise eternally begotten. We define besides that the explanation of those words "Filioque" was lawfully and reasonably added to the Symbol for the sake of declaring the truth, and under the necessity then pressing. Likewise, that the body of Christ is truly confected in unleavened or in leavened wheaten bread, and that priests ought to confect the body of the Lord in either one of these, each according to the custom of his own Church, whether western or eastern.”
In nomine sanctae Trinitatis, Patris et Filii et Spiritus Sancti, hoc sacro universali approbante FLORENTINO Concilio diffinimus, ut haec fidei veritas ab omnibus Christianis credatur et suscipiatur, sicque omnes profiteantur, quod Spiritus Sanctus ex Patre et Filio aeternaliter est, et essentiam suam suumque esse subsistens habet ex Patre simul et Filio, et ex utroque aeternaliter tanquam ab uno principio et unica spiratione procedit; declarantes, quod id, quod sancti Doctores et Patres dicunt, ex Patre per Filium procedere Spiritum Sanctum, ad hanc intelligentiam tendit, ut per hoc significetur, Filium quoque esse secundum Graecos quidem causam, secundum Latinos vero principium subsistentiae Spiritus Sancti, sicut et Patrem. Et quoniam omnia, quae Patris sunt, Pater ipse unigenito Filio suo gignendo dedit praeter esse Patrem, hoc ipsum quod Spiritus Sanctus procedit ex Filio, ipse Filius a Patre aeternaliter habet a quo etiam aeternaliter genitus est. Diffinimus insuper explicationem verborum illorum «Filioque» veritatis declarandae gratia, et imminente tunc necessitate licite ac rationabiliter Symbolo fuisse appositam. Item in azymo sive fermentato pane triticeo corpus Christi veraciter confici sacerdotesque in altero ipsorum Domini corpus conficere debere, unumquemque scilicet iuxta suae Ecclesiae sive occidentalis, sive orientalis consuetudinem.
Source
Modern · 1953 →

The in-app commentary runs from the Fathers to the early-modern record, then stops — that's where the public-domain sources end, not where the reading does. For the modern reading, follow the sources directly.