Born into an aristocratic family ten years before the sack of Rome, Leo (c. 400 - 461) is singled out by the emperor to serve as a diplomatic envoy in settling a dispute in Gaul. While he is away, the bishop of Rome dies, and Leo is unanimously elected to fill the post. He secures power, insisting that popes are in a direct line of succession from the apostles and that anyone who rejected papal authority was not within the "body of Christ." He consolidates this authority by moving against heretics, particularly Pelagians and Manicheans.
Leo, in the judgment of many historians, is the first real pope. Not always specifying the head of the church, the term pope was used for bishops and as a broad term of respect for church officials. True papal supremacy is not clearly defined until the reign of Leo, coming to full bloom under Gregory I.
Leo's rule was theological as well as political. In 448, Leo receives a letter from Eutyches, an abbot in a monastery near Constantinople. Eutyches writes of the influence of the Nestorian heresy, but then he himself comes under fire for allegedly subscribing to the same heresy and is excommunicated by Bishop Flavian. He asks Leo to reinstate him, and when Leo fails to act, he is absolved in a "robber council," an action that is perceived to be a threat to papal power and is promptly annulled by Leo.
In 449 Leo writes a letter to Bishop Flavian. This "Tome of Leo" becomes a key document as the church continues to define orthodoxy at the Council of Chalcedon in 451. Here Leo's definition of the two natures of Christ is deemed the orthodox position. He accuses Eutyches of seeking to "dissolve Jesus" in his endeavor "to separate the human nature from him, and to make void by shameless inventions that mystery by which alone we have been saved." Leo charges Eutyches with thinking "the Lord's crucifixion to be unreal."
Eutyches, seventy years old and the head of a monastery of some three hundred monks, refuses to appear before Bishop Flavian, convinced that the deck is stacked against him. When he finally does appear and is questioned, he waffles on precisely what he is willing to confess. But the statement he makes leaves no doubt among the supporters of Leo that he is a heretic. "I confess that our Lord was of two natures before the union, but after the union I confess one nature."
Leo regards such a confession as blatant heresy, seeking to clarify the incarnation and the twofold nature of Christ with words that rise above dry dogma: "Without detriment therefore to the properties of either nature and substance which then came together in one person, majesty took on humility, strength weakness, eternity mortality."
Heresy is not the only matter weighing Leo down. Only a few years after the landmark Council of Chalcedon, he faces a desperate situation in Rome; barbarians again threaten to sack the city. Attila, nicknamed "the scourge of God," is making his way to Rome. One early account serves to establish Leo as "the Great" for the centuries that follow. According to the anonymous author, Attila "came into Italy, inflamed with fury . . . He was utterly cruel in inflicting torture, greedy in plundering, insolent in abuse." Leo stands strong, approaching Attila and saying, "We pray for mercy and deliverance. O Attila, thou king of kings . . . the people have felt thy scourge; now as suppliants they would feel thy mercy." This account records the appearance of Peter and Paul, who "threatened Attila with death if he did not obey the pope's command. Wherefore Attila . . . straightway promised a lasting peace and withdrew beyond the Danube."
That Leo served both as head of state and chief diplomat demonstrated the weakness of Imperial Rome since the events of 410. But his talking down Attila surely did not signal the end of the invasions of the city. Some years later, Vandal marauders moving northward from Africa pillaged the city despite Leo's pleas. For the next years, until his death in 461, he took charge of cleanup and restoration as well as ministering to those who had been taken captive to Africa.