In addition to Aragon, which James represented in person, representatives of the kings of Germany, England, Scotland, France, the Spains and Sicily were present, with procurators also representing the kingdoms of Norway, Sweden, Hungary, Bohemia, the "realm of Dacia" and the duchy of Poland. In the procedures to be observed in the council, for the first time the nations appeared as represented elements in an ecclesiastical council, as they had already become represented in the governing of medieval universities. This innovation marks a stepping-stone towards the acknowledgment of coherent ideas of nationhood, which were in the process of creating the European nation-states.
The main topics discussed at the council were the conquest of the Holy Land and the union of the Eastern and Western Churches. The first session took place on 7 May 1274 and was followed by five additional sessions on 18 May 1274, 4 or 7 June 1274, 6 July 1274, 16 July 1274, and 17 July 1274. By the end of the council, 31 constitutions were promulgated. In the second session, the fathers approved the decree Zelus fidei, which contained no juridical statutes but rather summed up constitutions about the perils of the Holy Land, the means for paying for a proposed crusade, the excommunication of pirates and corsairs and those who protected them or traded with them, a declaration of peace among Christians, a grant of an indulgence for those willing to go on crusade, restoration of communion with the Greeks, and the definition of the order and procedure to be observed in the council. The Greeks conceded on the issue of the Filioque (two words added to the Nicene creed), and union was proclaimed, but the union was later repudiated by Andronicus II, heir to Michael VIII. The council also recognized Rudolf I as Holy Roman Emperor, ending the Interregnum.
Cathedral of St John, Lyon, illuminated for a festival
Wishing to end the Great Schism that divided the Roman Catholic Church from the Eastern Orthodox Church, Gregory X had sent an embassy to Michael VIII Palaeologus, who had reconquered Constantinople, putting an end to the remnants of the Latin Empire in the East, and he asked Latin despots in the East to curb their ambitions. Eastern dignitaries arrived at Lyon on 24 June 1274 presenting a letter from the Emperor. On 29 June 1274 (the Feast of Peter and Paul, patronal feast of the popes), Gregory celebrated Mass in St John's Church where both sides took part. The Greeks read the Nicene Creed, with the Western addition of the Filioque clause sung three times. The council was seemingly a success, but did not provide a lasting solution to the schism; the Emperor was anxious to heal the schism, but the Eastern clergy opposed the decisions of the Council. Patriarch Joseph of Constantinople abdicated, and was replaced by John Bekkos, a convert to the cause of union. In spite of a sustained campaign by Bekkos to defend the union intellectually, and vigorous and brutal repression of opponents by Michael, the vast majority of Byzantine Christians remained implacably opposed to union with the Latin "heretics". Michael's death in December 1282 put an end to the union of Lyons. His son and successor Andronicus II repudiated the union in the Council of Blachernae (1285), and Bekkos was forced to abdicate, being eventually exiled and imprisoned under house arrest until his death in 1297.