In response to the question "Whether a man may merit anything from God?" St. Thomas writes:
It is written (Jeremiah 31:16): "There is a reward for thy work." Now a reward means something bestowed by reason of merit. Hence it would seem that a man may merit from God. [ST I-II, Q114, A1]Now what is this, especially given that it was earlier today that we saw Aquinas write that man could not merit any supernatural gifts at all? Well, the first thing that must be said of course is that perhaps St. Thomas has made a mistake; he is only human after all. Notwithstanding that possibility, though, it must be said that he is astonishingly consistent in his work, so that we are better off to not presuming that he has contradicted himself. Similarly, given his own (and that of his fellow scholastics) intensely conservative outlook on the truth - so that they prefer not to pursue novelties, but to adhere to the Faith delivered - we must surely presume that some explanation is forthcoming. And there is.
I answer that, Merit and reward refer to the same, for a reward means something given anyone in return for work or toil, as a price for it. Hence, as it is an act of justice to give a just price for anything received from another, so also is it an act of justice to make a return for work or toil. Now justice is a kind of equality, as is clear from the Philosopher (Ethic. v, 3), and hence justice is simply between those that are simply equal; but where there is no absolute equality between them, neither is there absolute justice, but there may be a certain manner of justice, as when we speak of a father's or a master's right (Ethic. v, 6), as the Philosopher says. And hence where there is justice simply, there is the character of merit and reward simply. But where there is no simple right, but only relative, there is no character of merit simply, but only relatively, in so far as the character of justice is found there, since the child merits something from his father and the slave from his lord.In other words, he means nothing different here than what we've previously seen, and what St. Augustine said:
Now it is clear that between God and man there is the greatest inequality: for they are infinitely apart, and all man's good is from God. Hence there can be no justice of absolute equality between man and God, but only of a certain proportion, inasmuch as both operate after their own manner. Now the manner and measure of human virtue is in man from God. Hence man's merit with God only exists on the presupposition of the Divine ordination, so that man obtains from God, as a reward of his operation, what God gave him the power of operation for, even as natural things by their proper movements and operations obtain that to which they were ordained by God; differently, indeed, since the rational creature moves itself to act by its free-will, hence its action has the character of merit, which is not so in other creatures. [ST, op. cit.; emphasis added]
no one merits the grace which brings them to attain to justice; our prayer is a gift; no one merits justification; no one merits Faith, which is a gift, without which no one does good; love is given by God; our merits are gifts effected through grace; when God rewards, He crowns His gifts, even the merits He has given; eternal life is a grace. [emphasis added]This post really adds nothing new to what we have seen, but I include it by way of addressing a part of the Summa that might possibly be quoted out of context by Aquinas' (and Catholics') detractors. The point remains the same: we cannot save ourselves; we cannot earn salvation.