21 You see how they all foretell the coming of the Lord. You see how they know the voice of the sparrow. Let us know what sort of voice this is. For the Lord Himself shall descend from heaven with a shout, with the voice of the Archangel, and with the trump of God. The Archangel shall make proclamation and say to all, Arise to meet the Lord. And fearful will be that descent of our Master. David says, God shall manifestly come, even our God, and shall not keep silence; a fire shall burn before Him, and a fierce tempest round about Him, and the rest. The Son of Man shall come to the Father, according to the Scripture which was just now read, on the clouds of heaven, drawn by a stream of fire, which is to make trial of men.
Then if any man's works are of gold, he shall be made brighter; if any man's course of life be like stubble, and unsubstantial, it shall be burnt up by the fire. And the Father shall sit, having His garment white as snow, and the hair of His head like pure wool. But this is spoken after the manner of men; wherefore? Because He is the King of those who have not been defiled with sins; for, He says, I will make your sins white as snow, and as wool, which is an emblem of forgiveness of sins, or of sinlessness itself. But the Lord who shall come from heaven on the clouds, is He who ascended on the clouds; for He Himself has said, And they shall see the Son of Man coming on the clouds of heaven, with power and great glory.
Source: Catechetical Lectures (New Advent)