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.