Objection 1. It would seem that Christ could not merit in the first instant of His conception. For the free-will bears the same relation to merit as to demerit. But the devil could not sin in the first instant of his creation, as was shown in I, 63, 5. Therefore neither could Christ's soul merit in the first instant of its creation--that is, in the first instant of Christ's conception.
Objection 2. Further, that which man has in the first instant of his conception seems to be natural to him: for it is in this that his natural generation is terminated. But we do not merit by what is natural to us, as is clear from what has been said in I-II, 109, 5; I-II, 114, 2. Therefore it seems that the use of free-will, which Christ as man had in the first instant of His conception, was not meritorious.
Objection 3. Further, that which a man has once merited he makes, in a way, his own: consequently it seems that he cannot merit the same thing again: for no one merits what is already his. If, therefore, Christ merited in the first instant of His conception, it follows that afterwards He merited nothing. But this is evidently untrue. Therefore Christ did not merit in the first instant of His conception.
On the contrary, Augustine [Paterius, Expos. Vet. et Nov. Test. super Ex. 40] says: "Increase of merit was absolutely impossible to the soul of Christ." But increase of merit would have been possible had He not merited in the first instant of His conception. Therefore Christ merited in the first instant of His conception.
I answer that, As stated above (Article 1), Christ was sanctified by grace in the first instant of His conception. Now, sanctification is twofold: that of adults who are sanctified in consideration of their own act; and that of infants who are sanctified in consideration of, not their own act of faith, but that of their parents or of the Church. The former sanctification is more perfect than the latter: just as act is more perfect than habit; and "that which is by itself, than that which is by another" [Aristotle, Phys. viii]. Since, therefore, the sanctification of Christ was most perfect, because He was so sanctified that He might sanctify others; consequently He was sanctified by reason of His own movement of the free-will towards God. Which movement, indeed, of the free-will is meritorious. Consequently, Christ did merit in the first instant of His conception.
Reply to Objection 1. Free-will does not bear the same relation to good as to evil: for to good it is related of itself, and naturally; whereas to evil it is related as to a defect, and beside nature. Now, as the Philosopher says (De Coelo ii, text. 18): "That which is beside nature is subsequent to that which is according to nature; because that which is beside nature is an exception to nature." Therefore the free-will of a creature can be moved to good meritoriously in the first instant of its creation, but not to evil sinfully; provided, however, its nature be unimpaired.
Reply to Objection 2. That which man has at the first moment of his creation, in the ordinary course of nature, is natural to him. but nothing hinders a creature from receiving from God a gift of grace at the very beginning of its creation. In this way did Christ's soul in the first instant of its creation receive grace by which it could merit. And for this reason is that grace, by way of a certain likeness, said to be natural to this Man, as explained by Augustine (Enchiridion xl).
Reply to Objection 3. Nothing prevents the same thing belonging to someone from several causes. And thus it is that Christ was able by subsequent actions and sufferings to merit the glory of immortality, which He also merited in the first instant of His conception: not, indeed, so that it became thereby more due to Him than before, but so that it was due to Him from more causes than before.
Source: Summa Theologica (New Advent)