'Peter has spoken through Leo'

"....By this means Eutyches, who seemed to be deserving of honour under the title of Presbyter, is now shown to be exceedingly thoughtless and sadly inexperienced, so that to him also we may apply the prophet’s words, “He refused to understand in order to act well:  he meditated unrighteousness on his bed.”  What, indeed, is more unrighteous than to entertain ungodly thoughts, and not to yield to persons wiser and more learned?  But into this folly do they fall who, when hindered by some obscurity from apprehending the truth, have recourse, not to the words of the Prophets, not to the letters of the Apostles, nor to the authority of the Gospels, but to themselves; and become teachers of error, just because they have not been disciples of the truth.  For what learning has he received from the sacred pages of the New and the Old Testament, who does not so much as understand the very beginning of the Creed?  And that which, all the world over, is uttered by the voices of all applicants for regeneration, is still not grasped by the mind of this aged man.  If, then, he knew not what he ought to think about the Incarnation of the Word of God, and was not willing, for the sake of obtaining the light of intelligence, to make laborious search through the whole extent of the Holy Scriptures, he should at least have received with heedful attention that general Confession common to all, whereby the whole body of the faithful profess that they “believe in God the Father Almighty, and in Jesus Christ his only Son our Lord, who was born of the Holy Ghost and the Virgin Mary.”  By which three clauses the engines of almost all heretics are shattered.  For when God is believed to be both “Almighty” and “Father,” it is proved that the Son is everlasting together with himself, differing in nothing from the Father, because he was born as “God from God,” Almighty from Almighty, Coeternal from Eternal; not later in time, not inferior in power, not unlike him in glory, not divided from him in essence, but the same Only-begotten and Everlasting Son of an Everlasting Parent was “born of the Holy Ghost and the Virgin Mary.”  This birth in time in no way detracted from, in no way added to, that divine and everlasting birth; but expended itself wholly in the work of restoring man, who had been deceived; so that it might both overcome death, and by its power “destroy the devil who had the power of death.”  For we could not have overcome the author of sin and of death, unless he who could neither be contaminated by sin, nor detained by death, had taken upon himself our nature, and made it his own.  For, in fact, he was “conceived of the Holy Ghost” within the womb of a Virgin Mother, who bore him as she had conceived him, without loss of virginity..."

from the 'Tome' * of Pope St Leo the Great, translated by William Bright.
("Select Sermons of S. Leo the Great on the Incarnation with his XXVIIIth Epistle called the “Tome.”  London, 1886)

* The letter sent by Pope Leo I in the year 449 to the beleaguered Patriarch of Constantinople, Flavianus, concerning the Church's teaching about the person of Christ. The letter affirms that Christ has two natures, human and divine, united in the one divine Person of the Son of God. At the Council of Chalcedon, two years later, the Tome was acclaimed as expressing the settled mind of the Church concerning the doctrine of the Incarnation with the declaration, "Peter has spoken through Leo."