According to some, this Saint was born in Rome, but according to others in Tyrrenia (Tuscany), and was consecrated to the archiepiscopal throne of Rome in 440. In 448, when Saint Flavian, Archbishop of Constantinople, summoned Eutyches, an archimandrite in Constantinople, to give account for his teaching that there was only one nature in Christ after the Incarnation, Eutyches appealed to Saint Leo in Rome. After Saint Leo had carefully examined Eutyches's teachings, he wrote an epistle to Saint Flavian, setting forth the Orthodox teaching of the person of Christ, and His two natures, and also counseling Flavian that, should Eutyches sincerely repent of his error, he should be received back with all good will. At the Council held in Ephesus in 449, which was presided over by Dioscorus, Patriarch of Alexandria (and which Saint Leo, in a letter to the holy Empress Pulcheria in 451, was the first to call "The Robber Council"), Dioscorus, having military might behind him, did not allow Saint Leo's epistle to Flavian to be read, although repeatedly asked to do so; even before the Robber Council was held, Dioscorus had uncanonically received the unrepentant Eutyches back into communion. Because Saint Leo had many cares in Rome owing to the wars of Attila the Hun and other barbarians, in 451 he sent four delegates to the Fourth Ecumenical Council, where 630 Fathers gathered in Chalcedon during the reign of Marcian, to condemn the teachings of Eutyches and those who supported him. Saint Leo's epistle to Flavian was read at the Fourth Council, and was confirmed by the Holy Fathers as the Orthodox teaching on the incarnate person of our Lord; it is also called the "Tome of Leo." The Saint wrote many works in Latin; he reposed in 461. See also Saint Anatolius, July 3.

This apostolic and prophetic man, and model of faith and truth, was a disciple of John the Evangelist, successor of Bucolus (Feb. 6), and teacher of Irenaeus (Aug. 23). He was an old man and full of days when the fifth persecution was raised against the Christians under Marcus Aurelius. When his pursuers, sent by the ruler, found Polycarp, he commanded that they be given something to eat and drink, then asked them to give him an hour to pray; he stood and prayed, full of grace, for two hours, so that his captors repented that they had come against so venerable a man. He was brought by the Proconsul of Smyrna into the stadium and was commanded, "Swear by the fortune of Caesar; repent, and say, 'Away with the atheists.'" By atheists, the Proconsul meant the Christians. But Polycarp, gazing at the heathen in the stadium, waved his hand towards them and said, "Away with the atheists." When the Proconsul urged him to blaspheme against Christ, he said: "I have been serving Christ for eighty-six years, and He has wronged me in nothing; how can I blaspheme my King Who has saved me?" But the tyrant became enraged at these words and commanded that he be cast into the fire, and thus he gloriously expired about the year 163. As Eusebius says, "Polycarp everywhere taught what he had also learned from the Apostles, which also the Church has handed down; and this alone is true" (Eccl. Hist., Book IV, ch. 14,15).