The Second Council of Lyons was one of the most largely attended of conciliar assemblies, there being present five hundred bishops, sixty abbots, more than a thousand prelates or procurators. Gregory X, who presided, had been a canon of Lyons; Peter of Tarentaise, who assisted as Cardinal-Bishop of Ostia, had been Archbishop of Lyons. It opened 7 May, 1274, in the church of St. John. There were five other sessions (18 May, 7 June, 6 July, 16 July, 17 July). At the second session Gregory X owing to the excessive numbers rejected the proxies of chapters, abbots, and unmitred priors, except those who had been summoned by name. Among those who attended the council were James I, King of Aragon, the ambassadors of the Kings of France and England, the ambassadors of the Emperor Michael Palaeologus and the Greek clergy, the ambassadors of the Khan of the Tatars. The conquest of the Holy Land and the union of the Churches were the two ideas for the realization of which Gregory X had convoked the council.
Despite the protest of Richard of Mapham, dean of Lincoln, he obtained that during the six years for the benefit of the crusade a tithe of all the benefices of Christendom should go to the pope, but when James I, King of Aragon, wished to organize the expedition at once the representatives of the Templars opposed the project, and a decision was postponed. Ambassadors of the Khan of Tatary arrived at Lyons, 4 July, to treat with Gregory X, who desired that during the war against Islam the Tatars should leave the Christians in peace. Two of the ambassadors were solemny baptized 16 July.
Gregory X had prepared for the union by sending in 1273 an embassy to Constantinople to Michael Palaeologus, and by inducing Charles, King of Sicily, and Philip, Latin Emperor of Constantinople, to moderate their political ambitions. On 24 June, 1274, there arrived at Lyons as representatives of Palaeologus, Germanus, Patriarch of Constantinople, Theophanes, Bishop of Nicea, Georgius Acropolita, senator and great logothete, Nicholas Panaretus, president of the ward-robe, Berrhoeota, chief interpreter, and Georgius Zinuchi. The letter from Palaeologus which they presented had been written in the name of fifty archbishops and five hundred bishops or synods. On 29 June, the feast of Sts. Peter and Paul, Gregory X celebrated Mass in the church of St. John, the Epistle, Gospel and Creed were read or sung in Latin and Greek, the article "qui a patre filioque procedit" was sung three times by the Greeks. On 6 July, after a sermon by Peter of Tarentaise and the public reading of the letter of Palaeologus, Georgius Acropolita and the other ambassadors promised fidelity to the Latin Church, abjured twenty-six propositions which it denied, and promised the protection of the emperor to the Christians of the Holy Land. Gregory X intoned the "Te "Deum", spoke on the text "Desiderio desideravi hoc pascha manducare vobiscum", and on 28 July wrote joyful letters to Michael, to his son Andronicus, and forty-one metropolitans. Three letters dated February, 1274, written to the pope by Michael and Andronicus, in which they recognized his supremacy, exist as proofs of the emperor's good faith, despite the efforts to throw doubt on it by means of a letter of Innocent V (1276) which seems to point to the conclusion that Georgius Acropolita, who at the council had promised fidelity to the Roman Church, had not been expressly authorized by the emperor.
The Council of Lyons dealt also with the reform of the Church, in view of which Gregory X in 1273 had addressed questions to the bishops and asked of Hubert de Romans, the former general of the Friars Preachers, a certain programme for discussion and of John of Vercelli, the new general of the order, a draft of formal constitutions. Henri of Gölder, Bishop of Liège, Frederick, Abbot of St. Paul without the Walls, the Bishops of Rhodes and of Würzburg were deposed for unworthiness, and certain mendicant orders were suppressed. The council warmly approved the two orders of St. Dominic and St. Francis. Fearing the opposition of the King of Spain who had in his kingdom three religious military orders, the idea was abandoned of forming all military orders into one. Gregory X, to avoid a repetition of the too lengthy vacancies of the papal see, caused it to be decided that the cardinals should not leave the conclave till the pope had been elected. This constitution which inflicted certain material privations on the cardinals if the election was too long delayed, was suspended in 1276 by Adrian V, and a few months later revoked by John XXI, but was re-established later in many of its articles, and is even yet the basis of legislation on the conclaves. Lastly the Council of Lyons dealt with the vacancy of the imperial throne. James I of Aragon pretended to it; Gregory X removed him and on 6 June Rudolph I was proclaimed King of the Romans and future emperor. Such was the work of the council during which died the two greatest doctors of the Middle Ages. St. Thomas Aquinas, summoned by the pope, died at Frosinone (7 March, 1274) on his way to Lyons. St. Bonaventure, after important interviews at the Council with the Greek ambassadors, died 15 July, at Lyons, and was praised by Peter of Tarentaise, the future Innocent V, in a touching funeral sermon.
MARTIN, "Bullaire et Conciles de Lyon" (Lyon, 1905) (excellent); MANSI, "Coll Conciliorum", XXIII, 605-82, XXIV, 37-136; HEFELE, "History of Christian Councils", tr. CLARK; HAVET, "Biobliotheque de l'Ecole des Chartes", XLVI, 1855, 233-50; BERGER, "Registres d'Innocent IV (in course of publication); GUIRAUD AND CADIER, "Registres de Gregoire X et Jean XXI (in course of publication).