Objection 1. It would seem that temporal nativity is not to be attributed to Christ. For "to be born is a certain movement of a thing that did not exist before it was born, which movement procures for it the benefit of existence" [Cf. Augustine, De Unit. Trin. xii]. But Christ was from all eternity. Therefore He could not be born in time.
Objection 2. Further, what is perfect in itself needs not to be born. But the Person of the Son of God was perfect from eternity. Therefore He needs not to be born in time. Therefore it seems that He had no temporal birth.
Objection 3. Further, properly speaking, nativity regards the person. But in Christ there is only one person. Therefore in Christ there is but one nativity.
Objection 4. Further, what is born by two nativities is born twice. But this proposition is false; "Christ was born twice": because the nativity whereby He was born of the Father suffers no interruption; since it is eternal. Whereas interruption is required to warrant the use of the adverb "twice": for a man is said to run twice whose running is interrupted. Therefore it seems that we should not admit a double nativity in Christ.
On the contrary, Damascene says (De Fide Orth. iii): "We confess two nativities in Christ: one of the Father--eternal; and one which occurred in these latter times for our sake."
I answer that, As stated above (Article 1), nature is compared to nativity, as the terminus to movement or change. Now, movement is diversified according to the diversity of its termini, as the Philosopher shows (Phys. v). But, in Christ there is a twofold nature: one which He received of the Father from eternity, the other which He received from His Mother in time. Therefore we must needs attribute to Christ a twofold nativity: one by which He was born of the Father from all eternity; one by which He was born of His Mother in time.
Reply to Objection 1. This was the argument of a certain heretic, Felician, and is solved thus by Augustine (Contra Felic. xii). "Let us suppose," says he, "as many maintain, that in the world there is a universal soul, which, by its ineffable movement, so gives life to all seed, that it is not compounded with things begotten, but bestows life that they may be begotten. Without doubt, when this soul reaches the womb, being intent on fashioning the passible matter to its own purpose, it unites itself to the personality thereof, though manifestly it is not of the same substance; and thus of the active soul and passive matter, one man is made out of two substances. And so we confess that the soul is born from out the womb; but not as though, before birth, it was nothing at all in itself. Thus, then, but in a way much more sublime, the Son of God was born as man, just as the soul is held to be born together with the body: not as though they both made one substance, but that from both, one person results. Yet we do not say that the Son of God began thus to exist: lest it be thought that His Divinity is temporal. Nor do we acknowledge the flesh of the Son of God to have been from eternity: lest it be thought that He took, not a true human body, but some resemblance thereof."
Reply to Objection 2. This was an argument of Nestorius, and it is thus solved by Cyril in an epistle [Cf. Acta Concil. Ephes., p. 1, cap. viii]: "We do not say that the Son of God had need, for His own sake, of a second nativity, after that which is from the Father: for it is foolish and a mark of ignorance to say that He who is from all eternity, and co-eternal with the Father, needs to begin again to exist. But because for us and for our salvation, uniting the human nature to His Person, He became the child of a woman, for this reason do we say that He was born in the flesh."
Reply to Objection 3. Nativity regards the person as its subject, the nature as its terminus. Now, it is possible for several transformations to be in the same subject: yet must they be diversified in respect of their termini. But we do not say this as though the eternal nativity were a transformation or a movement, but because it is designated by way of a transformation or movement.
Reply to Objection 4. Christ can be said to have been born twice in respect of His two nativities. For just as he is said to run twice who runs at two different times, so can He be said to be born twice who is born once from eternity and once in time: because eternity and time differ much more than two different times, although each signifies a measure of duration.
Source: Summa Theologica (New Advent)