The Advent mystery is a mystery of beginning: but it is also the mystery of an end. The fullness of time is the end of all that was not yet fullness. It is the completion of all that was still incomplete, all that was still partial. It is the fulfillment in oneness of all that was fragmentary.
The Advent mystery in our own lives is the beginning of the end of all, in us, that is not yet Christ. It is the beginning of the end of unreality. And that is surely a cause of joy! But unfortunately we cling to our unreality, we prefer the part to the whole, we continue to be fragments, we do not want to be "one man in Christ."
Theologically, since human nature was assumed by the Word of God in Christ, all humanity is at least potentially "the humanity of Christ" in the sense that every existing human nature belongs by right, and indeed in fact, to Christ. Hence the terrible truth that a mankind which belongs to Christ without perhaps knowing it, or without being able to really evaluate the meaning of so astonishing a mystery, is spiritually alienated from him and is tearing itself to pieces.
The Body of Adam ("Man"), which should be the Body of God's Love, is torn with hate. The Body of Adam which should be transfigured with light, is a Body of obscurity and untruth. That which should be One in love is divided into millions of frenzied and murderous hostilities. Yet the fact remains: Christ the King of Peace has come into the world and saved it. He has saved Man, He has established His Kingdom, and His Kingdom is the Kingdom of Peace. Furthermore, we are His Kingdom.
(Thomas Merton: «Advent: Hope or Delusion» in Meditations on Liturgy (Oxford: Mowbray, 1976), p. 48-49)