This is taken from Leo I, Sermons 73-74. I have cut some sentences and paragraphs, smoothed out the translation at some points, and added a few phrases and sentences to clarify Leo’s thought.

During the time between the Lord’s Resurrection and Ascension, God had one aim in view: To teach and impress upon both the eyes and hearts of His own people that the Lord Jesus Christ was as truly risen, as He was truly born, suffered, and died. Hence the most blessed Apostles and all the disciples, who had been both bewildered at His death on the cross and slow in believing His Resurrection, were so strengthened by the clarity of the truth that when the Lord entered the heights of heaven, not only were they affected with no sadness, but were filled with great joy. Truly great and unspeakable was their cause for joy, when human nature went up above the dignity of all heavenly creatures, to pass above the angels’ ranks and to rise beyond the archangels’ heights. The exaltation of our nature reached no limit until, received to sit with the Eternal Father, it was set on the throne to share His glory, to whose nature our nature was united in the Son.

Since then Christ’s Ascension is our uplifting, and the hope of the Body is raised up to the same place where the glory of the Head has gone before, let us exult with joy and delight in the loyal giving of thanks. Today, not only are we confirmed as possessors of paradise, but in Christ we have penetrated the heights of heaven, and have gained still greater things through Christ’s unspeakable grace than we had lost through the devil’s malice. Our enemy drove us from the happiness of our first home, but the Son of God has made us members of Himself and placed us at the right hand of the Father.

As therefore at the Easter commemoration, the Lord’s Resurrection was the cause of our rejoicing; so the subject of our present gladness is His Ascension, as we commemorate that day on which our humble nature was raised in Christ above all the host of heaven, over all the ranks of angels, beyond the height of all powers, to sit with God the Father. In this way, God’s grace becomes more wondrous, since, though Jesus was removed from our sight, faith did not fail, hope did not waver, love did not grow cold. Rather, by His absence, our faith grows stronger. How should godliness spring up in our hearts, or how should a man be justified by faith, if our salvation rested on things that lie beneath our eyes?

Thomas doubted the Resurrection until he had tested by sight and touch the traces of His Passion in His very Flesh. But our Lord said to him, “because you have seen Me, you have believed: blessed are they who have not seen and yet have believed.” This is our blessedness, and we gain this blessing because Jesus has been removed from us. When He had fulfilled all things concerning the Gospel preaching and the mysteries of the New Testament, our Lord Jesus Christ, on the fortieth day after the Resurrection, in the presence of the disciples, was raised into heaven, and ended His presence with us in the body, to abide at the Father’s right hand until the times the Father has fore-ordained for multiplying the sons of the Church are accomplished, and He comes to judge the living and the dead in the same flesh in which He ascended. What was till then the visible presence of our Redeemer was changed into a sacramental presence; to make faith more excellent and stronger, sight gave way to faith.

This faith, increased by the Lord’s Ascension and established by the gift of the Holy Ghost, was not terrified by bonds, imprisonments, banishments, hunger, fire, attacks by wild beasts, refined torments of cruel persecutors. For this faith throughout the world men, women, beardless boys, even tender maids, fought to the shedding of their blood. This faith cast out spirits, drove off sicknesses, raised the dead. The blessed apostles themselves also, after being confirmed by so many miracles and instructed by so many di scourses, had yet been panic-stricken by the horrors of the Lord’s Passion and had not accepted the truth of His resurrection without hesitation. But they made such progress after the Lord’s Ascension that everything which had previously filled them with fear was turned into joy.

By His ascension, the Son of Man and Son of God attained a more excellent and holier fame, when He betook Himself back to the glory of the Father’s Majesty, and in an ineffable manner began to be nearer to the Father in respect of His Godhead, after having become farther away in respect of His manhood. A better instructed faith then began to understand the Son’s equality with the Father without the necessity of handling the corporeal substance of Christ, whereby He is less than the Father.

Hence comes that which the Lord said after His Resurrection, when Mary Magdalene, representing the Church, hastened to approach and touch Him: “Touch Me not, for I have not yet ascended to My Father.” That is to say, I would not have you come to Me as to a mer human body, nor yet recognize Me by fleshly perceptions: I put you off for higher things, I prepare greater things for you: when I have ascended to My Father, then you shall handle Me more perfectly and truly, for you shall grasp what you can not touch and believe what you can not see. When Jesus ascended from us to a great distance, at the same time He came yet nearer to us.

And so, dearly-beloved, let us rejoice with spiritual joy, and let us with gladness pay God worthy thanks. Let us not set our minds on things on the earth, but raise our hearts’ eyes unimpeded to those heights where Christ is, seated at the right hand of the Father. For fleshly pleasures wage war for the devil, whose delight it is to fetter souls that strive after things above, with the enticements of corruptible good things, and to draw them away from the place from which he himself has been banished. Against Satan’s plots every believer must keep careful watch that he may crush his foe at the point where the attack is made.

And there is no more powerful weapon against the devil’s wiles than kindly mercy and bounteous charity, by which every sin is either escaped or vanquished. This lofty power is not attained until that which is opposed to it has been overthrown. What is so hostile to mercy and works of charity as greed, which is the root of all evils? Unless that vice is destroyed by lack of nourishment, it will grow in the heart as an evil weed taking root, until the heart springs up with the thorns and briars of vices rather than the seed of true goodness. Let us then, dearly-beloved, resist this pestilence of evil and follow after charity, without which no virtue can flourish, that by this path of love whereby Christ came down to us, we too may rise up to Him, to Whom with God the Father and the Spirit be glory for ever and ever. Amen.