Objection 1. It would seem that the human action of Christ could not be meritorious to Him. For before His death Christ was a comprehensor even as He is now. But comprehensors do not merit: because the charity of the comprehensor belongs to the reward of beatitude, since fruition depends upon it. Hence it does not seem to be the principle of merit, since merit and reward are not the same. Therefore Christ before His passion did not merit, even as He does not merit now.
Objection 2. Further, no one merits what is due to him. But because Christ is the Son of God by nature, the eternal inheritance is due to Him, which other men merit by their works. And hence Christ Who, from the beginning, was the Word of God, could not merit anything for Himself.
Objection 3. Further, whoever has the principle does not properly merit what flows from its possession. But Christ has the glory of the soul, whence, in the natural course, flowed the glory of the body, as Augustine says (Ep. ad Dios cxviii); though by a dispensation it was brought about that in Christ the glory of the soul should not overflow to the body. Hence Christ did not merit the glory of the body.
Objection 4. Further, the manifestation of Christ's excellence is a good, not of Christ Himself, but of those who know Him. Hence it is promised as a reward to such as love Christ that He will be manifested to them, according to John 14:21: "He that loveth Me, shall be loved of My Father, and I will love him and will manifest Myself to him." Therefore Christ did not merit the manifestation of His greatness.
On the contrary, The Apostle says (Philippians 2:8-9): "Becoming obedient unto death... For which cause God also hath exalted Him." Therefore by obeying He merited His exaltation and thus He merited something for Himself.
I answer that, To have any good thing of oneself is more excellent than to have it from another, for "what is of itself a cause is always more excellent than what is a cause through another," as is said Phys. viii, 5. Now a thing is said to have, of itself, that of which it is to some extent the cause. But of whatever good we possess the first cause by authority is God; and in this way no creature has any good of itself, according to 1 Corinthians 4:7: "What hast thou that thou hast not received?" Nevertheless, in a secondary manner anyone may be a cause, to himself, of having certain good things, inasmuch as he cooperates with God in the matter, and thus whoever has anything by his own merit has it, in a manner, of himself. Hence it is better to have a thing by merit than without merit.
Now since all perfection and greatness must be attributed to Christ, consequently He must have by merit what others have by merit; unless it be of such a nature that its want would detract from Christ's dignity and perfection more than would accrue to Him by merit. Hence He merited neither grace nor knowledge nor the beatitude of His soul, nor the Godhead, because, since merit regards only what is not yet possessed, it would be necessary that Christ should have been without these at some time; and to be without them would have diminished Christ's dignity more than His merit would have increased it. But the glory of the body, and the like, are less than the dignity of meriting, which pertains to the virtue of charity. Hence we must say that Christ had, by merit, the glory of His body and whatever pertained to His outward excellence, as His Ascension, veneration, and the rest. And thus it is clear that He could merit for Himself.
Reply to Objection 1. Fruition, which is an act of charity, pertains to the glory of the soul, which Christ did not merit. Hence if He merited by charity, it does not follow that the merit and the reward are the same. Nor did He merit by charity inasmuch as it was the charity of a comprehensor, but inasmuch as it was that of a wayfarer. For He was at once a wayfarer and a comprehensor, as was said above (Question 15, Article 10). And therefore, since He is no longer a wayfarer, He is not in the state of meriting.
Reply to Objection 2. Because by nature Christ is God and the Son of God, the Divine glory and the lordship of all things are due to Him, as to the first and supreme Lord. Nevertheless a glory is due to Him as a beatified man; and this He has partly without merit, and partly with merit, as is clear from what has been said.
Reply to Objection 3. It is by Divine appointment that there is an overflow of glory from the soul to the body, in keeping with human merit; so that as man merits by the act of the soul which he performs in the body, so he may be rewarded by the glory of the soul overflowing to the body. And hence not only the glory of the soul, but also the glory of the body falls under merit, according to Romans 8:11: "He... shall quicken also our [Vulgate: 'your'] mortal bodies, because of His Spirit that dwelleth in us [Vulgate: 'you']." And thus it could fall under Christ's merit.
Reply to Objection 4. The manifestation of Christ's excellence is His good as regards the being which it has in the knowledge of others; although in regard to the being which they have in themselves it chiefly belongs to the good of those who know Him. Yet even this is referred to Christ inasmuch as they are His members.
Source: Summa Theologica (New Advent)