On this, the Memorial of St. Leo the Great (c. 400-461), a selection from Pope Benedict XVI's general audience of March 5, 2008:
We are familiar with Pope Leo's action thanks to his most beautiful sermons - almost 100 in a splendid and clear Latin have been 
preserved - and thanks to his approximately 150 letters. In these texts the Pontiff appears in all his greatness, devoted to the service of truth in charity through an assiduous exercise of the Word which shows him to us as both
Theologian and Pastor. Leo the Great, constantly thoughtful of his faithful and of the people of Rome but also of communion between the different Churches and of their needs, was a tireless champion and upholder of the Roman Primacy, presenting himself as the Apostle Peter's authentic heir: the many Bishops who gathered at the Council of Chalcedon, the majority of whom came from the East, were well aware of this.
This Council, held in 451 and in which 350 Bishops took part, was the most important assembly ever to have been celebrated in the history of the Church. Chalcedon represents the sure goal of the Christology of the three previous Ecumenical Councils: Nicea in 325, Constantinople in 381 and Ephesus in 431. By the sixth century these four Councils that sum up the faith of the ancient Church were already being compared to the four Gospels. This is what Gregory the Great affirms in a famous letter (I, 24): "I confess that I receive and revere, as the four books of the Gospel so also the four Councils", because on them, Gregory explains further, "as on a four-square stone, rises the structure of the holy faith". The Council of Chalcedon, which rejected the heresy of Eutyches who denied the true human nature of the Son of God, affirmed the union in his one Person, without confusion and without separation, of his two natures, human and divine.
The Pope asserted this faith in Jesus Christ, true God and true man, in an important doctrinal text addressed to the Bishop of Constantinople, the so-called Tome to Flavian which, read at Chalcedon, was received by the Bishops present with an eloquent acclamation. Information on it has been preserved in the proceedings of the Council: "Peter has spoken through the mouth of Leo", the Council Fathers announced in unison. From this intervention in particular, but also from others made during the Christological controversy in those years, it is clear that the Pope felt with special urgency his responsibilities as Successor of Peter, whose role in the Church is unique since "to one Apostle alone was entrusted what was communicated to all the Apostles", as Leo said in one of his sermons for the Feast of Sts Peter and Paul (83, 2). And the Pontiff was able to exercise these responsibilities, in the West as in the East, intervening in various circumstances with caution, firmness and lucidity through his writings and legates. In this manner he showed how exercising the Roman Primacy was as necessary then as it is today to effectively serve communion, a characteristic of Christ's one Church. ...
In particular, Leo the Great taught his faithful - and his words still apply for us today - that the Christian liturgy is not the memory of past events, but the actualization of invisible realities which act in the lives of each one of us. This is what he stressed in a sermon (cf. 64, 1-2) on Easter, to be celebrated in every season of the year "not so much as something of the past as rather an event of the present". All this fits into a precise project, the Holy Pontiff insisted: just as, in fact, the Creator enlivened with the breath of rational life man formed from the dust of the ground, after the original sin he sent his Son into the world to restore to man his lost dignity and to destroy the dominion of the devil through the new life of grace.
This is the Christological mystery to which St Leo the Great, with his Letter to the Council of Ephesus, made an effective and essential contribution, confirming for all time - through this Council - what St Peter said at Caesarea Philippi. With Peter and as Peter, he professed: "You are the Christ, the Son of the living God". And so it is that God and man together "are not foreign to the human race but alien to sin" (cf. Serm. 64). Through the force of this Christological faith he was a great messenger of peace and love. He thus shows us the way: in faith we learn charity. Let us therefore learn with St Leo the Great to believe in Christ, true God and true Man, and to implement this faith every day in action for peace and love of neighbour.
Here are a few excerpts from St. Leo the Great's famed sermons:
On Peter and the Papacy:
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. (Sermon III)
On Arianism:
This union, dearly beloved, whereby the Creator is joined to the creature, Arian blindness could not see with the eyes of intelligence, but, not believing that the Only-begotten of God was of the same glory and substance with the Father, spoke of the Son's Godhead as inferior, drawing its arguments front those words which are to be referred to the "form of a slave," in 
respect of which, in order to show that it belongs to no other or different person in Himself, the same Son of God with the same form, says, "The Father is greater than I," just as He says with the same form, "I and my Father are one." For in "the form of a slave," which He took at the end of the ages for our restoration, He is inferior to the Father: but in the form of God, in which He was before the ages, He is equal to the Father. In His human humiliation He was "made of a woman, made under the Law:" in His Divine majesty He abides the Word of God, "through whom all things were made." Accordingly, He Who in the form of God made man, in the form of a slave was made man. For both natures retain their own proper character without loss: and as the form of God did not do away with the form of a slave, so the form of a slave did not impair the form of God. And so the mystery of power united to weakness, in respect of the same human nature, allows the Son to be called inferior to the Father: but the Godhead, which is One in the Trinity of the Father, Son, and Holy Ghost, excludes all notion of inequality. For the eternity of the Trinity has nothing temporal, nothing dissimilar in nature: Its will is one, Its substance identical, Its power equal, and yet there are not three GodS, but one God; because it is a true and inseparable unity, where there can be no diversity. Thus in the whole and perfect nature of true man was true God born, complete in what was His own, complete in what was ours. And by "ours" we mean what the Creator formed in us from the beginning, and what He undertook to repair. For what the deceiver brought in, and man deceived committed, had no trace in the Saviour; nor because He partook of man's weaknesses, did He therefore share our faults. He took the form of a slave without stain of sin, increasing the human and not diminishing the divine: for that "emptying of Himself," whereby the Invisible made Himself visible, was the bending down of pity, not the failing of power. (Sermon XXIII)
On Man Partaking in the Birth of Christ:
And so to undo this chain of sin and death, the Almighty Son of God, that fills all things and contains,all things, altogether equal to the Father and co-eternal in one essence from Him and with Him, took on Him man's nature, and the Creator and Land of all things deigned to be a mortal: choosing for His mother one whom He had made, one who, without loss of her maiden honour, supplied so much of bodily substance, that without the pollution of human seed the New Man might be possessed of purity and truth. In Christ, therefore, born of the Virgin's womb, the nature does not differ from ours, because His nativity is wonderful. For He Who is true God, is also true man: and there is no lie in either nature. "The Word became flesh" by exaltation of the flesh, not by failure of the Godhead: which so tempered its power and goodness as to exalt our nature by taking it, and not to lose His own by imparting it. In this nativity of Christ, according to the prophecy of David, "truth sprang out of the earth, and righteousness looked down from heaven." In this nativity also, Isaiah's saying is fulfilled, "let the earth produce and bring forth salvation, and let righteousness spring up together." For the earth of human flesh, which in the first transgressor, was cursed, in this Offspring of the Blessed Virgin only produced a seed that was blessed and free from the fault of its stock. And each one is a partaker of this spiritual origin in regeneration; and to every one when he is re-born, the water of baptism is like the Virgin's womb; for the same Holy Spirit fills the font, Who filled the Virgin, that the sin, which that sacred conception overthrew, may be taken away by this mystical washing. (Sermon XXIV)
The Triumph of the Cross:
The Cross of Christ, therefore, symbolize the true altar of prophecy, on which the oblation of man's nature should be celebrated by means of a salvation-bringing Victim. There the blood of the spotless Lamb blotted out the consequences of the ancient trespass: there the whole tyranny of the devil's hatred was crushed, and humiliation triumphed gloriously over the lifting up of pride: for so swift was the effect of Faith that of the robbers crucified with Christ, the one who believed in Christ as the Son of God entered paradise justified. Who can unfold the mystery of so great a boon? who can state the power of so wondrous a change? In a moment of thee the guilt of long evil-doing is done away; clinging to the cross, amid the cruel tortures of his struggling soul, he passes over to Christ; and to him, on whom his own wickedness had brought punishment, Christ's grace now gives a crown. (Sermon LV)
The Creed:
In that rule of Faith, dearly-beloved, which we have received in the very beginning of the Creed, on the authority of apostolic teaching, we acknowledge our Lord Jesus Christ, whom we call the only Son of God the Father Almighty, to be also born of the Virgin Mary by the Holy Ghost. Nor do we reject His Majesty when we express our belief in His crucifixion, death, and resurrection on the third day. For all that is God's and all that is Man's are simultaneously fulfilled by His Manhood and His Godhead, so that in virtue of the union of the Possible with the Impossible, His power cannot be affected by His weakness, nor His weakness overcome by His power. And rightly was the blessed Apostle Peter praised for confessing this union, who when the Lord was inquiring what the disciples knew of Him, quickly anticipated the rest and said, "Thou art Christ, the Son of the living God[2]." And this assuredly he saw, not by the revelation of flesh or blood, which might have hindered his inner sight, but by the very Spirit of the Father working in his believing heart, that in preparation for ruling the whole Church he might first learn what he would have to teach, and for the solidification of the Faith, which he was destined to preach, might receive the assurance, "Thou art Peter, and upon this rock I will build My Church, and the gates of hell shall not prevail against it[3]." The strength, therefore, of the Christian Faith, which, built upon an impregnable rock, fears not the gates of death, acknowledges the one Lord Jesus Christ to be both true God and true Man, believing Him likewise to be the Virgin's Son, Who is His Mother's Creator: born also at the end of the ages, though He is the Creator of time: Lord of all power, and yet one of mortal stock: ignorant of sin, and yet sacrificed for sinners after the likeness of sinful flesh. (Sermon LXII)
The Trinity:
But although, dearly-beloved, the actual form of the thing done was exceeding wonderful, and undoubtedly in that exultant chorus of all human languages the Majesty of the Holy Spirit was present, yet no one must think that His Divine substance appeared in what was seen with bodily eyes. For His Nature, which is invisible and shared in common with the Father and the Son, showed the character of His gift and work by the outward sign that pleased Him, but kept His essential property within His own Godhead: because human sight can no more perceive the Holy Ghost than it can the Father or the Son. For in the Divine Trinity nothing is unlike or unequal, and all that can be thought concerning Its substance admits of no diversity either in power or glory or eternity. And while in the property of each Person the Father is one, the Son is another, and the Holy Ghost is another, yet the Godhead is not distinct and different; for whilst the Son is the Only begotten of the Father, the Holy Spirit is the Spirit of the Father and the Son, not in the way that every creature is the creature of the Father and the Son, but as living and having power with Both, and eternally subsisting of That Which is the Father and the Son. And hence when the Lord before the day of His Passion promised the coming of the Holy Spirit to His disciples, He said, "I have yet many things to say to you, but ye cannot bear them now. But when He, the Spirit of Truth shall have come, He shall guide you into all the Truth. For He shall not speak from Himself, but whatsoever He shall have heard, He shall speak and shall announce things to come unto you. All things that the Father hath are Mine: therefore said I that He shall take of Mine, and shall announce it to you." Accordingly, there are not some things that are the Father's, and other the Son's, and other the Holy Spirit's: but all things whatsoever the Father has, the Son also has, and the Holy Spirit also has: nor was there ever a time when this communion did not exist, because with Them to have all things is to always exist. In them let no times, no grades, no differences be imagined, and, if no one can explain that which is true concerning God, let no one dare to assert what is not true. For it is more excusable not to make a full statement concerning His ineffable Nature than to frame an actually wrong definition. And so whatever loyal hearts can conceive of the Father's eternal and unchangeable Glory, let them at the same time understand it of the Son and of the Holy Ghost without any separation or difference. For we confess this blessed Trinity to be One God for this reason, because in these three Persons there is no diversity either of substance, or of power, or of will, or of operation. (Sermon LXXV)
