Objection 1. It would seem that Christ should have Himself made His birth known. For "a direct cause is always of greater power than an indirect cause," as is stated Phys. viii. But Christ made His birth known through others--for instance, to the shepherds through the angels, and to the Magi through the star. Much more, therefore, should He Himself have made His birth known.
Objection 2. Further, it is written (Sirach 20:32): "Wisdom that is hid and treasure that is not seen; what profit is there in them both?" But Christ had, to perfection, the treasure of wisdom and grace from the beginning of His conception. Therefore, unless He had made the fulness of these gifts known by words and deeds, wisdom and grace would have been given Him to no purpose. But this is unreasonable: because "God and nature do nothing without a purpose" (De Coelo i).
Objection 3. Further, we read in the book De Infantia Salvatoris that in His infancy Christ worked many miracles. It seems therefore that He did Himself make His birth known.
On the contrary, Pope Leo says (Serm. xxxiv) that the Magi found the "infant Jesus in no way different from the generality of human infants." But other infants do not make themselves known. Therefore it was not fitting that Christ should Himself make His birth known.
I answer that, Christ's birth was ordered unto man's salvation, which is by faith. But saving faith confesses Christ's Godhead and humanity. It behooved, therefore, Christ's birth to be made known in such a way that the proof of His Godhead should not be prejudicial to faith in His human nature. But this took place while Christ presented a likeness of human weakness, and yet, by means of God's creatures, He showed the power of the Godhead in Himself. Therefore Christ made His birth known, not by Himself, but by means of certain other creatures.
Reply to Objection 1. By the way of generation and movement we must of necessity come to the imperfect before the perfect. And therefore Christ was made known first through other creatures, and afterwards He Himself manifested Himself perfectly.
Reply to Objection 2. Although hidden wisdom is useless, yet there is no need for a wise man to make himself known at all times, but at a suitable time; for it is written (Sirach 20:6): "There is one that holdeth his peace because he knoweth not what to say: and there is another that holdeth his peace, knowing the proper time." Hence the wisdom given to Christ was not useless, because at a suitable time He manifested Himself. And the very fact that He was hidden at a suitable time is a sign of wisdom.
Reply to Objection 3. The book De Infantia Salvatoris is apocryphal. Moreover, Chrysostom (Hom. xxi super Joan.) says that Christ worked no miracles before changing the water into wine, according to John 2:11: "'This beginning of miracles did Jesus.' For if He had worked miracles at an early age, there would have been no need for anyone else to manifest Him to Israelites; whereas John the Baptist says (John 1:31): 'That He may be made manifest in Israel; therefore am I come baptizing with water.' Moreover, it was fitting that He should not begin to work miracles at an early age. For people would have thought Incarnation to be unreal, and, out of sheer spite, would have crucified Him before the proper time."
Source: Summa Theologica (New Advent)