First, here is a short excerpt from "The Ascension: The Beginning of a New Nearness," from Images of Hope: Meditations on Major Feasts (Ignatius, 2006):
You are surely familiar with all those precious, naïve images in which only the feet of Jesus are visible, sticking out of the cloud, at the heads of the apostles. The cloud, for its part, is a dark circle on the perimeter; on the inside, however, blazing light. It occurs to me that precisely in the apparent naïveté of this representation something very deep comes into view. All we see of Christ in the time of history are his feet and the cloud. His feet—what are they?
We are reminded, first of all, of a peculiar sentence from the Resurrection account in Matthew's Gospel, where it is said that the women held onto the feet of the Risen Lord and worshipped him. As the Risen One, he towers over earthly proportions. We can still only touch his feet; and we touch them in adoration. Here we could reflect that we come as worshippers, following his trail, close to his footsteps. Praying, we go to him; praying, we touch him, even if in this world, so to speak, always only from below, only from afar, always only on the trail of his earthly steps. At the same time it becomes clear that we do not find the footprints of Christ when we look only below, when we measure only footprints and want to subsume faith in the obvious. The Lord is movement toward above, and only in moving ourselves, in looking up and ascending, do we recognize him.
When we read the Church Fathers something important is added. The correct ascent of man occurs precisely where he learns, in humbly turning toward his neighbor, to bow very deeply, down to his feet, down to the gesture of the washing of feet. It is precisely humility, which can bow low, that carries man upward. This is the dynamic of ascent that the feast of the Ascension wants to teach us.
And this from "The Threefold Presence of Christ", from You Crown the Year With Your Goodness: Sermons Throught the Liturgical Year (Ignatius, 1989), by Fr. Hans Urs von Balthasar:
[Christ's] disappearance from the world begins with his Passion and ends with his Ascension. For since he was laid in the tomb, no worldly person, no one who lacks the Spirit of Christ, has seen him anymore. His coming to us, however, starts on Easter morning, where he meets one disciple after another; it continues throughout the Forty Days and is brought to its fulfillment at Pentecost, when he pours out his Spirit over the Church and thus fills her with his own innermost being. It is not that his presence changes into his absence; what changes is the mode of his presence. ...
As Catholics we can try to view this new presence from three angles: from God's point of view, from Christ's point of view and from the Church's point of view. ...
The Lord shares in God's mode of presence, but he is not only God, he is also man for all eternity, with a human body and a human soul. Now this humanity explicitly participates in the new mode of his presence and indwelling. And this is the really astonishing and baffling thing: that this finite soul and this limited body can share in the limitless omnipresence and intimacy of God. His wisdom and love have brought this miracle about: it is called "Eucharist". It is not only a spiritual being-together in which the parties think of one another, nor is it simply the kind of presence whereby man is in God: it is an indwelling of the divine-human being of Christ, soul and body, in the whole person, body in body and soul in soul. "He who eats my Flesh and drinks my Blood abides in me and I in him." ...
[Christ] is with us not only as God, not only as the eucharistic God-man, but also, essentially, as Church. What we mean by "Church" comes into being as a result of his Eucharist, from the outpouring of his Spirit; the Church lives in the power of his being-with-us all the days, to the end of the world. ...
Love is heaven on earth. Only thus is the mystery of the Ascension complete, in which the Son comes to us so that we may be where he is. He is with God, and God is love. And if we love, says John, we are with God and in God.
Finally, from my article, "The Image of Man Has Been Raised Up: On the Feast of the Ascension of the Lord":
At the Ascension the crucified, risen Son of God returns to His Father. Having descended to dusty earth, He now returns to heavenly glory. Having conquered death, He ascends to eternal life. But He returns to the right hand of the Father not just as the Word, but as the Incarnate Word. The doors of heaven are now open and humanity can now approach the throne room of God, the way having been paved by the life, death, and resurrection of the God-man. Pentecost, finally, is the manifestation of the God-man's Church, which is both human and divine. The Church was revealed to the world on that day—fifty days after Easter—by the power of the Holy Spirit.
All of this theology is nice enough, but what does it mean for us? It means the Feast of the Ascension is a celebration of salvation won. The Catholic Encyclopedia notes that "in the Eastern Church this feast was known as analepsis, the taking up, and also as the episozomene, the salvation, denoting that by ascending into His glory Christ completed the work of our redemption." The tendency is often to think of the Resurrection as the culmination of Jesus' salvific work, but it is the Ascension that places the final stamp of approval on the sacrificial and victorious work of our Savior. This is beautifully expressed in the first chapter of Paul's epistle to the Ephesians:
May the eyes of your hearts be enlightened, that you may know what is the hope that belongs to his call, what are the riches of glory in his inheritance among the holy ones, and what is the surpassing greatness of his power for us who believe, in accord with the exercise of his great might: which he worked in Christ, raising him from the dead and seating him at his right hand in the heavens ... (Eph. 1:17-20).
"The ascension of Christ is our elevation," declared Leo the Great in a sermon on the Ascension, "Hope for the body is also invited where the glory of the Head preceded us. Let us exult, dearly beloved, with worthy joy and be glad with a holy thanksgiving. Today we not only are established as possessors of paradise, but we have even penetrated the heights of the heavens in Christ." Where the sin of the first Adam closed the gates of Paradise, the righteousness of the new Adam has opened them wide.