22 But what is the sign of His coming? Lest a hostile power dare to counterfeit it. And then shall appear, He says, the sign of the Son of Man in heaven. Now Christ's own true sign is the Cross; a sign of a luminous Cross shall go before the King, plainly declaring Him who was formerly crucified: that the Jews who before pierced Him and plotted against Him, when they see it, may mourn tribe by tribe, saying, “This is He who was buffeted, this is He whose face they spat on, this is He whom they bound with chains, this is He whom of old they crucified, and set at nought. Whither, they will say, shall we flee from the face of Your wrath?”
But the Angel hosts shall encompass them, so that they shall not be able to flee anywhere. The sign of the Cross shall be a terror to His foes; but joy to His friends who have believed in Him, or preached Him, or suffered for His sake. Who then is the happy man, who shall then be found a friend of Christ? That King, so great and glorious, attended by the Angel-guards, the partner of the Father's throne, will not despise His own servants. For that His Elect may not be confused with His foes, He shall send forth His Angels with a great trumpet, and they shall gather together His elect from the four winds. He despised not Lot, who was but one; how then shall He despise many righteous? Come, you blessed of My Father, will He say to them who shall then ride on chariots of clouds, and be assembled by Angels.
Source: Catechetical Lectures (New Advent)