The prophecy of Christ's Godhead and Manhood, contained in the verse of Isaiah just now cited, is unfolded, and its force in refuting various heresies demonstrated.
54. This beginning did Isaiah see, and therefore he says: “A Child is born, a Son is given to us,” as also did the Magi, and therefore worshipped they, when they saw the little One in the stable, and said: “A Child is born,” and, when they saw the star, declared, “A Son is given to us.” On the one hand, a gift from earth— on the other, a gift from heaven— and both are One Person, perfect in respect of each, without any changeableness in the Godhead, as without any taking away from the fullness of the Manhood. One Person did the Magi adore, to one and the same they offered their gifts, to show that He Who was seen in the stall was the very Lord of heaven.
55. Mark how the two verbs differ in their import: “A Child is born, a Son is given.” Though born of the Father, yet is He not born, but given to us, forasmuch as the Son is not for our sakes, but we for the Son's. For indeed He was not born to us, being born before us, and the maker of all things created: nor is He now brought to life for the first time, Who was always, and was in the beginning; on the other hand, that which before-time was not is born to us. Again we find it thus recorded, how that the angel, when he spoke to the shepherds, said that He had been born: “Who is this day born to us a Saviour, Who is Christ the Lord, in the city of David.” To us, then, was born that which was not before— that is, a child of the Virgin, a body from Mary— for this was made after man had been created, whereas [the Godhead] was before us.
56. Some manuscripts read as follows: “A Child is born to us a Son is given to us;” that is to say, He, Who is Son of God, is born as Mary's child for us, and given to us. As for the fact that He is “given,” listen to the prophet's words: “And grant us Your salvation.” But that which is above us is given: what is from heaven is given: even as indeed we read concerning the Spirit, that “the love of God is shed abroad in our hearts by the Holy Spirit, Who is given unto us.”
57. But note how this passage is as water upon fire to a crowd of heresies. “A Child is born to us,” not to the Jews; “to us,” not to the Manichæans; “to us,” not to the Marcionites. The prophet says “to us,” that is, to those who believe, not to unbelievers. And He indeed, in His pitifulness, was born for all, but it is the disloyalty of heretics that has brought it to pass that the birth of Him Who was born for all should not profit all. For the sun is bidden to rise upon the good and the bad, but to them that see not there is no appearance of sunrise.
58. Even as the Child, then, is born not unto all, but unto the faithful: so the Son is given to the faithful and not to the unbelieving. He is given to us, not to the Photinians; for they affirm that the Son of God was not given unto us, but was born and first began to exist among us. To us is He given, not to the Sabellians, who will not hear of a Son being given, maintaining that Father and Son are one and the same. Unto us is He given, not unto the Arians, in whose judgment the Son was not given for salvation, but sent over subject and inferior, to whom, moreover, He is no “Counsellor,” inasmuch as they hold that He knows nought of the future, no Son, since they believe not in His eternity, though of the Word of God it is written: “That which was in the beginning;” and again: “In the beginning was the Word.” To return to the passage we set before us to discuss. “In the beginning,” says the Scripture, “before He made the earth, before He made the deeps, before He brought forth the springs of water, before all the hills He begot Me.”
Source: On the Christian Faith (De fide) (New Advent)