Papal authority: Decree of Valentinian - Leo was a significant contributor to the centralisation of spiritual authority within the Church and in reaffirming papal authority. While the bishop of Rome had always been viewed as the chief patriarch in the Western church, much of the pope's authority was delegated to local diocesan bishops. Not without serious opposition did he succeed in reasserting his authority in Gaul. Patroclus of Arles (d. 426) had received from Pope Zosimus the recognition of a subordinate primacy over the Gallican Church which was strongly asserted by his successor Hilary of Arles. An appeal from Chelidonius of Besançon gave Leo the opportunity to reassert the pope's authority over Hilary, who defended himself stoutly at Rome, refusing to recognize Leo's judicial status. Feeling that the primatial rights of the bishop of Rome were threatened, Leo appealed to the civil power for support, and obtained from Valentinian III the famous decree of June 6, 445, which recognized the primacy of the bishop of Rome based on the merits of Peter, the dignity of the city, and the Nicene Creed (in their interpolated form); ordained that any opposition to his rulings, which were to have the force of ecclesiastical law, should be treated as treason; and provided for the forcible extradition by provincial governors of anyone who refused to answer a summons to Rome. Faced with this decree, Hilary submitted to the pope, although under his successor, Ravennius, Leo divided the metropolitan rights between Arles and Vienne (450). -- Dispute with Dioscorus of Alexandria: In 445, Leo disputed with Pope Dioscorus, St. Cyril's successor as Pope of Alexandria, insisting that the ecclesiastical practice of his see should follow that of Rome on the basis that Mark the Evangelist, the disciple of Saint Peter and founder of the Alexandrian Church, could have had no other tradition than that of the prince of the apostles. This, of course, was not the position of the Copts, who saw the ancient patriarchates as equals. -- Council of Chalcedon: A favorable occasion for extending the authority of Rome in the East was offered in the renewal of the Christological controversy by Eutyches, who in the beginning of the conflict appealed to Leo and took refuge with him on his condemnation by Flavian. But on receiving full information from Flavian, Leo took his side decisively. In 451 at the Council of Chalcedon, after Leo's Tome on the two natures of Christ was read out, the bishops participating in the Council cried out: "This is the faith of the fathers ... Peter has spoken thus through Leo ..." -- Battling heresies:
An uncompromising foe of heresy, Leo found that in the diocese of Aquileia, Pelagians were received into church communion without formal repudiation of their errors; he wrote to rebuke them, making accusations of culpable negligence, and required a solemn abjuration before a synod. Manicheans fleeing before the Vandals had come to Rome in 439 and secretly organized there; Leo learned of this around 443, and proceeded against them by holding a public debate with their representatives, burning their books, and warning the Roman Christians against them. Nor was his attitude less decided against the Priscillianists. Bishop Turrubius of Astorga, astonished at the spread of this sect in Spain, had addressed the other Spanish bishops on the subject, sending a copy of his letter to Leo, who took the opportunity to exercise Roman policy in Spain. He wrote an extended treatise (21 July 447), against the sect, examining its false teaching in detail, and calling for a Spanish general council to investigate whether it had any adherents in the episcopate, but this was prevented by the political circumstances of Spain. -- On Dignity and Equality: In his Nativitate Domini, in the Christmas Day sermon "Christian, Remember your Dignity" Leo appears to articulate a fundamental and inclusive human dignity and equality: The saint, the sinner, and the unbeliever are all equal as sinners, and none is excluded in the call to "happiness": "Our Saviour, dearly-beloved, was born today: let us be glad. For there is no proper place for sadness, when we keep the birthday of the Life, which destroys the fear of mortality and brings to us the joy of promised eternity. No one is kept from sharing in this happiness. There is for all one common measure of joy, because as our Lord the destroyer of sin and death finds none free from charge, so is He come to free us all. Let the saint exult in that he draws near to victory. Let the sinner be glad in that he is invited to pardon. Let the gentile take courage in that he is called to life."