St. Thomas on Justification - "all merit is repugnant to grace"

St. Thomas denies that a man can merit the first grace (that by which he is justified).

The nature of grace is repugnant to reward of works, according to Romans 4:4: "Now to him that worketh, the reward is not reckoned according to grace but according to debt." Now a man merits what is reckoned to him according to debt, as the reward of his works. Hence a man may not merit the first grace.

I answer that, The gift of grace may be considered in two ways: first in the nature of a gratuitous gift, and thus it is manifest that all merit is repugnant to grace, since as the Apostle says (Romans 11:6), "if by grace, it is not now by works." Secondly, it may be considered as regards the nature of the thing given, and thus, also, it cannot come under the merit of him who has not grace, both because it exceeds the proportion of nature, and because previous to grace a man in the state of sin has an obstacle to his meriting grace, viz. sin. But when anyone has grace, the grace already possessed cannot come under merit, since reward is the term of the work, but grace is the principle of all our good works, as stated above (109 [discussed here, here, here, and here --RdP]). But if anyone merits a further gratuitous gift by virtue of the preceding grace, it would not be the first grace. Hence it is manifest that no one can merit for himself the first grace. [ST I-II, Q114, A5; bold in original, italics added]

Here we see St. Thomas emphasize first that merit is directly contrary to the very meaning of grace, and consequently grace ceases to be grace if it is merited. He reminds us that sin is a further obstacle to any pretense of meriting grace - one that by itself would utterly prevent such merit even if the two weren't contraries. And he tells us again that grace is the principle of any merit we may have subsequent to being justified.