Objection 1. It would seem that those to whom Christ's birth was made known were not suitably chosen. For our Lord (Matthew 10:5) commanded His disciples, "Go ye not into the way of the Gentiles," so that He might be made known to the Jews before the Gentiles. Therefore it seems that much less should Christ's birth have been at once revealed to the Gentiles who "came from the east," as stated Matthew 2:1.
Objection 2. Further, the revelation of Divine truth should be made especially to the friends of God, according to Job 36:33: "He sheweth His friend concerning it." But the Magi seem to be God's foes; for it is written (Leviticus 19:31): "Go not aside after wizards [magi], neither ask anything of soothsayers." Therefore Christ's birth should not have been made known to the Magi.
Objection 3. Further, Christ came in order to set free the whole world from the power of the devil; whence it is written (Malachi 1:11): "From the rising of the sun even to the going down, My name is great among the Gentiles." Therefore He should have been made known, not only to those who dwelt in the east, but also to some from all parts of the world.
Objection 4. Further, all the sacraments of the Old Law were figures of Christ. But the sacraments of the Old Law were dispensed through the ministry of the legal priesthood. Therefore it seems that Christ's birth should have been made known rather to the priests in the Temple than to the shepherds in the fields.
Objection 5. Further, Christ was born of a Virgin-Mother, and was as yet a little child. It was therefore more suitable that He should be made known to youths and virgins than to old and married people or to widows, such as Simeon and Anna.
On the contrary, It is written (John 13:18): "I know whom I have chosen." But what is done by God's wisdom is done becomingly. Therefore those to whom Christ's birth was made known were suitably chosen.
I answer that, Salvation, which was to be accomplished by Christ, concerns all sorts and conditions of men: because, as it is written (Colossians 3:11), in Christ "there is neither male nor female, [These words are in reality from Galatians 3:28] neither Gentile nor Jew... bond nor free," and so forth. And in order that this might be foreshadowed in Christ's birth, He was made known to men of all conditions. Because, as Augustine says in a sermon on the Epiphany (32 de Temp.), "the shepherds were Israelites, the Magi were Gentiles. The former were nigh to Him, the latter far from Him. Both hastened to Him together as to the cornerstone." There was also another point of contrast: for the Magi were wise and powerful; the shepherds simple and lowly. He was also made known to the righteous as Simeon and Anna; and to sinners, as the Magi. He was made known both to men, and to women--namely, to Anna--so as to show no condition of men to be excluded from Christ's redemption.
Reply to Objection 1. That manifestation of Christ's birth was a kind of foretaste of the full manifestation which was to come. And as in the later manifestation the first announcement of the grace of Christ was made by Him and His Apostles to the Jews and afterwards to the Gentiles, so the first to come to Christ were the shepherds, who were the first-fruits of the Jews, as being near to Him; and afterwards came the Magi from afar, who were "the first-fruits of the Gentiles," as Augustine says (Serm. 30 de Temp. cc.).
Reply to Objection 2. As Augustine says in a sermon on the Epiphany (Serm. 30 de Temp.): "As unskilfulness predominates in the rustic manners of the shepherd, so ungodliness abounds in the profane rites of the Magi. Yet did this Corner-Stone draw both to Itself; inasmuch as He came 'to choose the foolish things that He might confound the wise,' and 'not to call the just, but sinners,'" so that "the proud might not boast, nor the weak despair." Nevertheless, there are those who say that these Magi were not wizards, but wise astronomers, who are called Magi among the Persians or Chaldees.
Reply to Objection 3. As Chrysostom says [Hom. ii in Matth. in the Opus Imperf., among the supposititious works of Chrysostom]: "The Magi came from the east, because the first beginning of faith came from the land where the day is born; since faith is the light of the soul." Or, "because all who come to Christ come from Him and through Him": whence it is written (Zechariah 6:12): "Behold a Man, the Orient is His name." Now, they are said to come from the east literally, either because, as some say, they came from the farthest parts of the east, or because they came from the neighboring parts of Judea that lie to the east of the region inhabited by the Jews. Yet it is to be believed that certain signs of Christ's birth appeared also in other parts of the world: thus, at Rome the river flowed with oil [Eusebius, Chronic. II, Olymp. 185]; and in Spain three suns were seen, which gradually merged into one [Cf. Eusebius, Chronic. II, Olymp. 184].
Reply to Objection 4. As Chrysostom observes (Theophylact., Enarr. in Luc. ii, 8), the angel who announced Christ's birth did not go to Jerusalem, nor did he seek the Scribes and Pharisees, for they were corrupted, and full of ill-will. But the shepherds were single-minded, and were like the patriarchs and Moses in their mode of life. Moreover, these shepherds were types of the Doctors of the Church, to whom are revealed the mysteries of Christ that were hidden from the Jews.
Reply to Objection 5. As Ambrose says (on Luke 2:25): "It was right that our Lord's birth should be attested not only by the shepherds, but also by people advanced in age and virtue": whose testimony is rendered the more credible by reason of their righteousness.
Source: Summa Theologica (New Advent)