8 A special synod was called at Constantinople; St. Flavian, bishop of that city, presided; Eutyches, who had been vigorously spreading his errors throughout the monasteries, was accused of heresy by Bishop Eusebius of Dorylaeum, and condemned. He considered that an injury had been done to him who had withstood the growth of the Nestorian heresy, and appealed to the judgment of some of the bishops placed in higher authority. And so St. Leo the Great, bishop of the Apostolic See, also received letters of appeal of this kind. No one could have been more suitable and capable for the refutation of Eutyches's error. His solid and shining virtues, his zealous watch equally over peace and religion, his strenuous defense of the dignity of the Roman see, his skill in the spoken word and equally in the management of affairs, have won for him the admiration of all succeeding ages. Moreover, he was accustomed in his allocutions and letters to maintain with great piety and pious greatness that the mystery of the one person and the two natures in Christ could never be preached sufficiently. 'The Catholic Church lives by this faith, and is nourished by it, that in Jesus Christ neither is the humanity believed without the true Divinity, nor the divinity without true humanity' (St. Leo the Great, Ep . xxviii, 5. PL . liv, 777).
Source: Sempiternus Rex Christus (Vatican.va)