Jesus said to his disciples:
“There will be signs in the sun, the moon, and the stars, and on earth nations will be in dismay, perplexed by the roaring of the sea and the waves. People will die of fright in anticipation of what is coming upon 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. But when these signs begin to happen, stand erect and raise your heads because your redemption is at hand.
“Beware that your hearts do not become drowsy from carousing and drunkenness and the anxieties of daily life, and that day catch you by surprise like a trap. For that day will assault everyone who lives on the face of the earth. Be vigilant at all times and pray that you have the strength to escape the tribulations that are imminent and to stand before the Son of Man.” Luke 21:25-28, 34-36
The misfortunes that befall Jerusalem are signs of what will happen before the Son of Man comes: all creations (heavens, earth, and sea) will be affected by the distress of nations (v. 25), their fear and foreboding. Christians, however, won’t feel this distress; they will hold their heads high (v. 28), because Christ’s victory (v. 27) is their victory, too. They will see how well-founded their hope was when they bore trials patiently (21:10-19): my brothers, we must be patient and persevere, so that having been given hope of truth and freedom. We may embrace truth and freedom themselves. […] Let no one abandon excellent works through impatience or, overcome by temptation, renounce the excellent life halfway down the path; he would lose the fruit of what he had done because he did not finish what he had begun” (St. Cyprian, De bono patientiae, 13 and 15).
However, the events having to do with the end (of which the destruction of the city is a foretaste and a symbol) will happen without any warning (v. 35). The fact that Jerusalem did fall should serve as a warning to be alert, for the “Son of Man” could come at any time; hence the final exhortation, to “watch at all times”: if we pray (v. 36)and live a mild life (v. 34) we will be ready for Jesus when he comes (see 21:28): “Let us be mild in order to dedicate our soul to prayer, persevere in fasting and make our prayers to God who sees all things. […] Let us imitate his patience and if we must suffer for his name, give him glory, for that is the example he gave us, that is what we believe” (Saint Polycarp, Ad Philippenses, 7-8).