From: Luke 21:20-28
Discourse on the Destruction of Jerusalem
and the End of the World (Continuation)
(Jesus said to his disciples),  "But when you see Jerusalem surrounded by armies, then know that its desolation has come near.  Then let those who are in Judea flee to the mountains, and let those who are inside the city depart, and let not those who are out in the country enter it;  for these are days of vengeance, to fulfill all that is written.  Alas for those who are with child and for those who give suck in those days! For great distress shall be upon the earth and wrath upon this people;  they shall be upon the earth and wrath upon this people;  they will fall by the edge of the sword, and be led captive among all nations; and Jerusalem will be trodden down by the Gentiles, until the times of the Gentiles are fulfilled.
 "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.
20-24. Jesus gives quite a detailed prophecy of the destruction of the Holy City. When the Christians living there saw the armies getting closer, they remembered this prophecy and fled to Transjordan (cf. Eusebius, "Ecclesiastical History," III, 5). Christ had advised them to flee as soon as possible because this is the time when God would punish Jerusalem for its sins, as the Old Testament predicted (Is 5:5-6).
Catholic tradition sees Israel as symbolizing the Church. In fact, in the Book of Revelation the Church triumphant is called the heavenly Jerusalem (cf. Rev 21:2). Therefore, by applying this passage to the Church, the sufferings the Holy City experiences can symbolize the contradictions the pilgrim Church will experience due to the sins of men, for "she herself takes her place among the creatures which groan and travail yet and await the revelation of the children of God" (Vatican II, "Lumen Gentium", 48).
24. "The times of the Gentiles" means the period in which the Gentiles, who do not belong to the Jewish people, will become menbers of the new people of God, the Church, until the Jews themselves are converted at the end of the world (cf. Rom 11:11-32).
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).
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.