Objection 1. It would seem that Christ's birth should not have been made known to anyone. For, as stated above (1, ad 3), it befitted the salvation of mankind that Christ's first coming should be hidden. But Christ came to save all; according to 1 Timothy 4:10: "Who is the Saviour of all men, especially of the faithful." Therefore Christ's birth should not have been made known to anyone.
Objection 2. Further, before Christ was born, His future birth was made known to the Blessed Virgin and Joseph. Therefore it was not necessary that it should be made known to others after His birth.
Objection 3. Further, no wise man makes known that from which arise disturbance and harm to others. But, when Christ's birth was made known, disturbance arose: for it is written (Matthew 2:3) that "King Herod, hearing" of Christ's birth, "was troubled, and all Jerusalem with him." Moreover, this brought harm to others; because it was the occasion of Herod's killing "all the male children that were in Bethlehem... from two years old and under." Therefore it seems unfitting for Christ's birth to have been made known to anyone.
On the contrary, Christ's birth would have been profitable to none if it had been hidden from all. But it behooved Christ's birth to be profitable: else He were born in vain. Therefore it seems that Christ's birth should have been made known to some.
I answer that, As the Apostle says (Romans 13:1) "what is of God is well ordered." Now it belongs to the order of Divine wisdom that God's gifts and the secrets of His wisdom are not bestowed on all equally, but to some immediately, through whom they are made known to others. Wherefore, with regard to the mystery of the Resurrection it is written (Acts 10:40-41): "God... gave" Christ rising again "to be made manifest, not to all the people, but to witnesses pre-ordained by God." Consequently, that His birth might be consistent with this, it should have been made known, not to all, but to some, through whom it could be made known to others.
Reply to Objection 1. As it would have been prejudicial to the salvation of mankind if God's birth had been made known to all men, so also would it have been if none had been informed of it. Because in either case faith is destroyed, whether a thing be perfectly manifest, or whether it be entirely unknown, so that no one can hear it from another; for "faith cometh by hearing" (Romans 10:17).
Reply to Objection 2. Mary and Joseph needed to be instructed concerning Christ's birth before He was born, because it devolved on them to show reverence to the child conceived in the womb, and to serve Him even before He was born. But their testimony, being of a domestic character, would have aroused suspicion in regard to Christ's greatness: and so it behooved it to be made known to others, whose testimony could not be suspect.
Reply to Objection 3. The very disturbance that arose when it was known that Christ was born was becoming to His birth. First, because thus the heavenly dignity of Christ is made manifest. Wherefore Gregory says (Hom. x in Evang.): "After the birth of the King of heaven, the earthly king is troubled: doubtless because earthly grandeur is covered with confusion when the heavenly majesty is revealed."
Secondly, thereby the judicial power of Christ was foreshadowed. Thus Augustine says in a sermon (30 de Temp.) on the Epiphany: "What will He be like in the judgment-seat; since from His cradle He struck terror into the heart of a proud king?"
Thirdly, because thus the overthrow of the devil's kingdom was foreshadowed. For, as Pope Leo says in a sermon on the Epiphany (Serm. v [Opus Imperfectum in Matth., Hom. ii, falsely ascribed to St. John Chrysostom): "Herod was not so much troubled in himself as the devil in Herod. For Herod thought Him to be a man, but the devil thought Him to be God. Each feared a successor to his kingdom: the devil, a heavenly successor; Herod, an earthly successor." But their fear was needless: since Christ had not come to set up an earthly kingdom, as Pope Leo says, addressing himself to Herod: "Thy palace cannot hold Christ: nor is the Lord of the world content with the paltry power of thy scepter." That the Jews were troubled, who, on the contrary, should have rejoiced, was either because, as Chrysostom says, "wicked men could not rejoice at the coming of the Holy one," or because they wished to court favor with Herod, whom they feared; for "the populace is inclined to favor too much those whose cruelty it endures."
And that the children were slain by Herod was not harmful to them, but profitable. For Augustine says in a sermon on the Epiphany (66 de Diversis): "It cannot be questioned that Christ, who came to set man free, rewarded those who were slain for Him; since, while hanging on the cross, He prayed for those who were putting Him to death."
Source: Summa Theologica (New Advent)