Joseph therefore was commanded to accept this boy about whom Isaiah had said, "For a boy has been born to you; a son has been given to you, whose rule has extended over his shoulders." Now he said "a son has been given to you" because Christ the Lord was born as a boy and was counted a son of Joseph and Mary. As to his going down into Egypt, Isaiah predicted this long before the fact when he said, "Behold, the Lord sits atop a swift cloud and will come into Egypt." By this statement the promise of the Lord's incarnation was clearly revealed. Since the Lord himself is invoked as "arising from on high, the sunlight of justice," it is right that he would come on a swift cloud. By this Isaiah means he would come in a hallowed body, a body weighed down by no sin and through which he covered the light of his own majesty with the envelope of the cloud of his body. Hosea as well points to this very fact when he says, "The king of Israel has been thrown down. Because Israel was small, I took delight in him. And I called my son from Egypt."19After Egypt's ancient, grave sin, after many blows had been divinely inflicted upon it, God the omnipotent Father, moved by devotion, sent his Son into Egypt. He did so that Egypt, which had long ago paid back the penalty of wickedness owed under Moses, might now receive Christ, the hope for salvation. How great was God's compassion as shown in the advent of his Son! Egypt, which of old under Pharaoh stood stubborn against God, now became a witness to and home for Christ. God's compassion toward Egypt was like that shown toward the magi, who deserved to know Christ the Lord. For, although the magi had for a long time dared resist the divine goodness under Moses, they now, having seen but a single star in heaven, believed in the Son of God. The cavalier magicians were handed over to punishment for their lack of faith. Others were brought to glory through faith, since they believed that God had been born in the flesh—God whom the Egyptian magicians were unwilling to recognize in all his divine excellence.
Source: TRACTATE ON MATTHEW 6.1