From: Luke 21:25-28, 34-36
Discourse on the Destruction of Jerusalem
and the End of the World (Continuation)
(Jesus said to his disciples),  "And there will be signs in sun and moon and stars, and upon the earth distress of nations in perplexity at the roaring of the sea and the waves,  men fainting with fear and foreboding of what is coming on the world; for the powers of the heavens will be shaken.  And then they will see the Son of Man coming in a cloud with power and great glory.  Now when these things begin to take place, look up and raise your heads, because your redemption is drawing near.
The Need for Vigilance
 "But take heed to yourselves lest your hearts be weighed down with dissipation and drunkenness and cares of this life, and that day come upon you suddenly like a snare;  for it will come upon all who dwell upon the face of the whole earth.  But watch at all times, praying that you may have strength to escape all these things that will take place, and to stand before the Son of Man."
25-26. Jesus refers to the dramatic changes in natural elements when the world is coming to an end. "The powers of the heavens will be shaken"; that is to say, the whole universe will tremble at the Lord's coming in power and glory.
27-28. Applying to himself the prophecy of Daniel (7:13-14), our Lord speaks of his coming in glory at the end of time. Mankind will see the power and glory of the Son of man, coming to judge the living and the dead. Christ will deliver this judgment in his human capacity. Sacred Scripture describes the solemnity of this event, when the sentence passed on each person in the particular judgment will be confirmed, and God's justice and mercy to men throughout history will shine out for all to see. "It was necessary not only that rewards should await the just and punishments the wicked, in the life to come, but that they should be awarded by a public and general judgment. Thus they will become better known and will be rendered more conspicuous to all, and a tribute of praise will be offered by all to the justice and providence of God" ("St Pius V Catechism", I, 8, 4).
This coming of the Lord is, then, a day of terror for evildoers and of joy for those who have remained faithful. The disciples should hold their heads high because their redemption is at hand. It is the day they will receive their reward. The victory won by Christ on the cross--victory over sin, over the devil and over death--will now be seen clearly, with all its implications. Therefore St Paul recommends that we be "awaiting our blessed hope, the appearing of the glory of our great God and Savior Jesus Christ" (Tit 2:13).
"He [Christ] ascended into heaven whence he will come again to judge the living and the dead, each according to his merits. Those who have responded to the love and compassion of God will go into eternal life. Those who have refused them to the end will be consigned to the fire that is never extinguished" (Paul VI, "Creed of the People of God", 12).
34-36. At the end of His discourse Jesus emphasizes that every Christian needs to be vigilant: we do not know the day nor the hour in which He will ask us to render an account of our lives. Therefore, we must at all times be trying to do God's will, so that death, whenever it comes, will find us ready. For those who act in this way, sudden death never takes them by surprise. As St. Paul recommends: "You are not in darkness, brethren, for that day to surprise you like a thief" (1 Thessalonians 5:4). Vigilance consists in making a constant effort not to be attached to the things of this world (the concupiscence of the flesh, the concupiscence of the eyes and pride of life: cf. 1 John 2:16) and in being assiduous in prayer, which keeps us close to God. If we live in this way, the day we die will be a day of joy and not of terror, for with God's help our vigilance will mean that our souls are ready to receive the visit of our Lord; they are in the state of grace: in meeting Christ we will not be meeting a judge who will find us guilty; instead He will embrace us and lead us into the house of His Father to remain there forever. "Does your soul not burn with the desire to make your Father-God happy when He has to judge you?" ([St] J. Escriva, "The Way", 746).
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland. Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.