Objection 1. It would seem that the Lord's coming to judgment will not be preceded by any signs. Because it is written (1 Thessalonians 5:3): "When they shall say: Peace and security; then shall sudden destruction come upon them." Now there would be no peace and security if men were terrified by previous signs. Therefore signs will not precede that coming.
Objection 2. Further, signs are ordained for the manifestation of something. But His coming is to be hidden; wherefore it is written (1 Thessalonians 5:2): "The day of the Lord shall come as a thief in the night." Therefore signs ought not to precede it.
Objection 3. Further, the time of His first coming was foreknown by the prophets, which does not apply to His second coming. Now no such signs preceded the first coming of Christ. Therefore neither will they precede the second.
On the contrary, It is written (Luke 21:25): "There shall be signs in the sun, and in the moon, and in the stars," etc.
Further, Jerome [St. Peter Damian, Opuscul. xlix; he quotes St. Jerome, but the reference is not known.] mentions fifteen signs preceding the judgment. He says that on the "first" day all the seas will rise fifteen cubits above the mountains; in the "second" day all the waters will be plunged into the depths, so that scarcely will they be visible; on the "third" day they will be restored to their previous condition; on the "fourth" day all the great fishes and other things that move in the waters will gather together and, raising their heads above the sea, roar at one another contentiously; on the "fifth" day, all the birds of the air will gather together in the fields, wailing to one another, with neither bite nor sup; on the "sixth" day rivers of fire will arise towards the firmament rushing together from the west to the east; on the "seventh" day all the stars, both planets and fixed stars, will throw out fiery tails like comets; on the "eighth" day there will be a great earthquake, and all animals will be laid low; on the "ninth" day all the plants will be bedewed as it were with blood; on the "tenth" day all stones, little and great, will be divided into four parts dashing against one another; on the "eleventh" day all hills and mountains and buildings will be reduced to dust; on the "twelfth" day all animals will come from forest and mountain to the fields, roaring and tasting of nothing; on the "thirteenth" day all graves from east to west will open to allow the bodies to rise again; on the "fourteenth" day all men will leave their abode, neither understanding nor speaking, but rushing hither and thither like madmen; on the "fifteenth" day all will die and will rise again with those who died long before.
I answer that, When Christ shall come to judge He will appear in the form of glory, on account of the authority becoming a judge. Now it pertains to the dignity of judicial power to have certain signs that induce people to reverence and subjection: and consequently many signs will precede the advent of Christ when He shall come to judgment, in order that the hearts of men be brought to subjection to the coming judge, and be prepared for the judgment, being forewarned by those signs. But it is not easy to know what these signs may be: for the signs of which we read in the gospels, as Augustine says, writing to Hesychius about the end of the world (Ep. lxxx), refer not only to Christ's coming to judgment, but also to the time of the sack of Jerusalem, and to the coming of Christ in ceaselessly visiting His Church So that, perhaps, if we consider them carefully, we shall find that none of them refers to the coming advent, as he remarks: because these signs that are mentioned in the gospels, such as wars, fears, and so forth, have been from the beginning of the human race: unless perhaps we say that at that time they will be more prevalent: although it is uncertain in what degree this increase will foretell the imminence of the advent. The signs mentioned by Jerome are not asserted by him; he merely says that he found them written in the annals of the Hebrews: and, indeed, they contain very little likelihood.
Reply to Objection 1. According to Augustine (Ad Hesych., Ep. lxxx) towards the end of the world there will be a general persecution of the good by the wicked: so that at the same time some will fear, namely the good, and some will be secure, namely the wicked. The words: "When they shall say: Peace and security," refer to the wicked, who will pay little heed to the signs of the coming judgment: while the words of Luke 21:26, "men withering away," etc., should be referred to the good.
We may also reply that all these signs that will happen about the time of the judgment are reckoned to occur within the time occupied by the judgment, so that the judgment day contains them all. Wherefore although men be terrified by the signs appearing about the judgment day, yet before those signs begin to appear the wicked will think themselves to be in peace and security, after the death of Antichrist and before the coming of Christ, seeing that the world is not at once destroyed, as they thought hitherto.
Reply to Objection 2. The day of the Lord is said to come as a thief, because the exact time is not known, since it will not be possible to know it from those signs: although, as we have already said, all these most manifest sings which will precede the judgment immediately may be comprised under the judgment day.
Reply to Objection 3. At His first advent Christ came secretly, although the appointed time was known beforehand by the prophets. Hence there was no need for such signs to appear at His first coming, as will appear at His second advent, when He will come openly, although the appointed time is hidden.
Source: Summa Theologica (New Advent)