Objection 1. It would seem that the union of Incarnation followed upon certain merits, because upon Psalm 32:22, "Let Thy mercy, o Lord, be upon us, as," etc. a gloss says: "Here the prophet's desire for the Incarnation and its merited fulfilment are hinted at." Therefore the Incarnation falls under merit.
Objection 2. Further, whoever merits anything merits that without which it cannot be. But the ancient Fathers merited eternal life, to which they were able to attain only by Incarnation; for Gregory says (Moral. xiii): "Those who came into this world before Christ's coming, whatsoever eminency of righteousness they may have had, could not, on being divested of the body, at once be admitted into the bosom of the heavenly country, seeing that He had not as yet come Who, by His own descending, should place the souls of the righteous in their everlasting seat." Therefore it would seem that they merited Incarnation.
Objection 3. Further, of the Blessed Virgin it is sung that "she merited to bear the Lord of all" [Little Office of B.V.M., Dominican Rite, Ant. at Benedictus], and this took place through Incarnation. Therefore Incarnation falls under merit.
On the contrary, Augustine says (De Praed. Sanct. xv): "Whoever can find merits preceding the singular generation of our Head, may also find merits preceding the repeated regeneration of us His members." But no merits preceded our regeneration, according to Titus 3:5: "Not by the works of justice which we have done, but according to His mercy He saved us, by the laver of regeneration." Therefore no merits preceded the generation of Christ.
I answer that, With regard to Christ Himself, it is clear from the above (Article 10) that no merits of His could have preceded the union. For we do not hold that He was first of all a mere man, and that afterwards by the merits of a good life it was granted Him to become the Son of God, as Photinus held; but we hold that from the beginning of His conception this man was truly the Son of God, seeing that He had no other hypostasis but that of the Son of God, according to Luke 1:35: "The Holy which shall be born of thee shall be called the Son of God." And hence every operation of this man followed the union. Therefore no operation of His could have been meritorious of the union.
Neither could the needs of any other man whatsoever have merited this union condignly: first, because the meritorious works of man are properly ordained to beatitude, which is the reward of virtue, and consists in the full enjoyment of God. Whereas the union of the Incarnation, inasmuch as it is in the personal being, transcends the union of the beatified mind with God, which is by the act of the soul in fruition; and therefore it cannot fall under merit. Secondly, because grace cannot fall under merit, for the principle of merit does not fall under merit; and therefore neither does grace, for it is the principle of merit. Hence, still less does Incarnation fall under merit, since it is the principle of grace, according to John 1:17: "Grace and truth came by Jesus Christ." Thirdly, because Incarnation is for the reformation of the entire human nature, and therefore it does not fall under the merit of any individual man, since the goodness of a mere man cannot be the cause of the good of the entire nature. Yet the holy Fathers merited Incarnation congruously by desiring and beseeching; for it was becoming that God should harken to those who obeyed Him.
And thereby the reply to the First Objection is manifest.
Reply to Objection 2. It is false that under merit falls everything without which there can be no reward. For there is something pre-required not merely for reward, but also for merit, as the Divine goodness and grace and the very nature of man. And again, the mystery of Incarnation is the principle of merit, because "of His fulness we all have received" (John 1:16).
Reply to Objection 3. The Blessed Virgin is said to have merited to bear the Lord of all; not that she merited His Incarnation, but because by the grace bestowed upon her she merited that grade of purity and holiness, which fitted her to be the Mother of God.
Source: Summa Theologica (New Advent)