from Ambrose's "Exposition of the Christian Faith", Book III, Chapters VIII and IX,
Nicene and Post-Nicene Fathers Series 2, Volume 10 (ed. Phillip Schaff):
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 fulness 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.
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...
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 amongst 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,” saith 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 begat Me...
Perchance you will ask how I came to cite, as referring to the Incarnation of Christ, the place, “The Lord created Me,” seeing that the creation of the universe took place before the Incarnation of Christ? But consider that the use of holy Scripture is to speak of things to come as though already past, and to make intimation of the union of two natures, Godhead and Manhood, in Christ, lest any should deny either His Godhead or His Manhood. In Isaiah, for example, you may read: “A Child is born unto us, and a Son is given unto us;” so here also [in the Proverbs] the prophet sets forth first the creation of the flesh, and joined thereto the declaration of the Godhead, that you might know that Christ is not two, but One, being both begotten of the Father before the worlds, and in the last times created of the Virgin. And thus the meaning is: I, Who am begotten before the worlds, am He Who was created of mortal woman, created for a set purpose.
Again, immediately before the declaration, “The Lord created Me,” He says, “I will tell of the things which are from eternity,” and before saying, “He begat,” He premised, “In the beginning, before He made the earth, before all hills.” In its extent, the preposition “before” reaches back into the past without end or limit, and so “Before Abraham was, I am,” clearly need not mean “after Adam,” just as “before the Morning Star”need not mean “after the angels.” But when He said “before,” He intended, not that He was included in any one’s existence, but that all things are included in His, for thus it is the custom of Holy Writ to show the eternity of God. Finally, in another passage you may read: “Before the mountains were brought forth, or ever the earth and the world were made, Thou art from everlasting to everlasting.” Before all created things, then, is the Son begotten; within all and for the good of all is He made; begotten of the Father, above the Law, brought forth of Mary, under the Law