The Second Council of Lyons

Michael VIII Palaiologos had recovered Constantinople for Byzantine rule in 1261, ending the Latin Empire the Fourth Crusade had installed there, but his restored empire remained militarily weak and faced renewed Western ambitions, particularly from Charles of Anjou, the king of Sicily, who had inherited a claim to Constantinople through the deposed Latin line. Union with Rome offered Michael a way to remove the religious justification Charles and others could use for a fresh crusade against a fellow Christian empire, and he pursued it as a matter of state policy well before Gregory X's council gave him the occasion to formalize it.

Byzantine representatives at Lyons, acting on Michael's instruction rather than on any broad consultation within the Byzantine church, recited the Nicene Creed with the disputed Filioque clause included and accepted papal primacy, and Gregory proclaimed the two churches reunited. The proclamation was received in Constantinople with open hostility from much of the clergy and populace, who had not been party to the negotiation and did not regard their emperor as competent to settle a theological question that had divided the churches for over two centuries on his own authority, and active resistance to the union persisted throughout Michael's remaining reign.

Gregory X owed his own election to a crisis the council was determined not to repeat. After Pope Clement IV died in 1268, the cardinals gathered to choose a successor took nearly three years to agree, a deadlock so prolonged that the citizens of Viterbo, where the cardinals were meeting, are said to have locked them into the episcopal palace and reduced their food to bread and water in an effort to force a decision, the origin of the word conclave, from the Latin for a room that can be locked with a key.

The Second Council of Lyons wrote a version of that improvised pressure into permanent church law: cardinals were now required to be sequestered together within ten days of a pope's death, denied contact with the outside world, and given progressively reduced rations the longer the election dragged on, rules intended to make a repeat of the Viterbo vacancy structurally difficult rather than merely embarrassing. The union with Constantinople proclaimed at the same council did not survive Michael VIII's death, but the conclave procedure did, and remains, with later modification, the basis of how the Catholic Church elects a pope today. Later councils tightened the procedure further, but the core principle Lyons established, that only the cardinals decide and that a supermajority is required, has governed every conclave since.