(17 Aug. 682–3 July 683)
A Sicilian, trained in the papal choir-school, admired for his eloquence, culture, and proficiency in Greek as well as Latin, he was probably elected in Jan. 681 but had to wait some eighteen months before receiving the imperial mandate necessary for his consecration. The letters from the guardians of the holy see announcing Agatho's death and Leo's election reached Constantinople on 10 Mar. 681, while the Sixth General Council was still in progress, but Emperor Constantine IV (668–85) deliberately held up ratification of the election until the council had formally anathematized Honorius I along with the other champions of monothelitism and Rome's acceptance of its decisions, including the anathema, was assured. The reluctance of the Roman envoys to accept the condemnation of a pope had to be overcome, and for this long and delicate negotiations, extending many months after the closure of the council (16 Sept. 681), were necessary. Only in July 682 did the envoys return to Rome, taking with them, along with the acts of the council, the mandate for Leo's consecration. Constantine, whose power to withhold this had been his trump card, showed his satisfaction by inviting the new pope to send a resident apocrisiarius to court, and by diminishing the tax burden on the papal patrimonies in Sicily and Calabria as well as the corn requisition for the army.
Leo was realist enough to accept the situation, which opened a period of peace and collaboration between Rome and Byzantium. He had the acts of the council, which condemned monothelitism, translated from Greek into Latin and, in implementation of imperial policy, took steps to circulate them to the church leaders and rulers of the west with letters calling for their subscription. His most important letter (7 May 683) was to Constantine, ratifying the council's decisions with the authority of Peter and anathematizing the monothelite leaders it had condemned, including Honorius I. In the original Latin text he spoke of him as having ‘attempted to subvert the pure faith by his profane betrayal’; in the Greek version this was softened to ‘by his betrayal he allowed the pure teaching to be sullied’. It is significant that, in his letter to the Spanish and, probably, other western bishops, he merely accused the dead pope of having failed through negligence to stamp out the flame of heresy. He was lenient too when Macarius I, deposed by the council (7 Mar. 681) as patriarch of Antioch, and other intransigent monothelites appeared in Rome, remitted by the emperor to him for judgement and sentence; with the exception of two, who recanted and were admitted to communion, he dispersed the rest among various monasteries.
A token of the new spirit of cooperation between emperor and pope was the definitive ending of Ravenna's short-lived bid for autocephalous status independent of Rome. Following a rapprochement made in Agatho's reign, Constantine revoked (682/3) the decree of Constans II (1 Mar. 666) granting Ravenna autonomy, and it was agreed that henceforth archbishops of that see should be consecrated by, and receive the pallium from, the pope. In return Leo exempted them from the fees traditionally incidental to consecration and from the obligation to come personally to Rome for the annual synod.