DOC 15 - Leo I, Sermon on the Petrine Doctrine

II. From Christ and through S. Peter the priesthood is handed on in perpetuity.
Although, therefore, dearly beloved, we be found both weak and slothful in fulfilling the duties of our office, because, whatever devoted and vigorous action we desire to do, we are hindered by the frailty of our very condition; yet having the unceasing propitiation of the Almighty and perpetual Priest, who being like us and yet equal with the Father, brought down His Godhead even to things human, and raised His Manhood even to things Divine, we worthily and piously rejoice over His dispensation, whereby, though He has delegated the care of His sheep to many shepherds, yet He has not Himself abandoned the guardianship of His beloved flock. And from His overruling and eternal protection we have received the support of the Apostles' aid also, which assuredly does not cease from its operation: and the strength of the foundation, on which the whole superstructure of the Church is reared, is not weakened(1) by the weight of the temple that rests upon it. For the solidity of that faith which was praised in the chief of the Apostles is perpetual: and as that remains which Peter believed in Christ, so that remains which Christ instituted in Peter. For when, as has been read in the Gospel lesson(2), the LORD had asked the disciples whom they believed Him to be amid the various opinions that were held, and the blessed Peter bad replied, saying, "Thou art the Christ, the Son of the living GOD," the LORD says, "Blessed art thou, Simon Bar-Jona, because flesh and flood hath not revealed it to thee, but My Father, which is in heaven. And I say to thee, that thou art Peter, and upon this rock will I build My church, and the gates of Hades shall not prevail against it. And I will give unto thee the keys of the kingdom of heaven. And whatsoever thou shall bind on earth, shall be bound in heaven; and whatsoever thou shall loose on earth, shall be loosed also in heaven."

III. S. Peter's work is still carried out by his successors.
The dispensation of Truth therefore abides, and the blessed Peter persevering in the strength of the Rock, which he has received, has not abandoned the helm of the Church, which he undertook. For he was ordained before the rest in such a way that from his being called the Rock, from his being pronounced the Foundation, from his being constituted the Doorkeeper of the kingdom of heaven, from his being set as the Umpire to bind and to loose, whose judgments shall retain their validity in heaven, from all these mystical titles we might know the nature of his association with Christ. And still to-day he more fully and effectually performs what is entrusted to him, and carries out every part of his duty and charge in Him and with Him, through Whom he has been glorified. And so if anything is rightly done and rightly decreed by us, if anything is won from the mercy of GOD by our daily supplications, it is of his work and merits whose power lives and whose authority prevails in his See. For this, dearly-beloved, was gained by that confession, which, inspired in the Apostle's heart by GOD the Father, transcended all the uncertainty of human opinions, and was endued with the firmness of a rock, which no assaults could shake. For throughout the Church Peter daily says, "Thou an the Christ, the Son of the living GOD," and every tongue which confesses the LORD, accepts the instruction his voice conveys. This Faith conquers the devil, and breaks the bonds of his prisoners. It uproots us from this earth and plants us in heaven, and the gates of Hades cannot prevail against it. For with such solidity is it endued by GOD that the depravity of heretics cannot mar it nor the unbelief of the heathen overcome it.

From: Medieval Sourcebook [http://www.fordham.edu/halsall/sbook.html]; trans. C.L. Feltoe, in Sermons of Leo The Great , in Library of Nicene and Post Nicene Fathers, 2nd Series, Vol. XII, (New York: 1895) [reprinted since by variety of publishers], p. 117