We have seen in a previous post that God’s justice requires that the evil He punishes must have been done willfully. In other words, man must have free will or he cannot justly be punished for his sins. It’s worth highlighting something else that I barely mentioned in that post: namely, that in St. Augustine’s view, God’s justice requires not merely that He punish the wicked, but also that He reward the good deeds of the righteous. On Free Choice of the Will isn’t specifically about justice nor about human merits, so there’s not much here to be seen on this topic, but I think it’s still merits noting in passing.
Also, if we admit that God is just (and it is sacrilege to deny this), He assigns rewards to the righteous and punishments to the wicked… (p. 3).
If evil is worthy of punishment, then it stands to reason that righteousness is worthy of reward. These are two sides of one coin.
This doesn’t mean that we are able, solely in our own strength, to merit reward from God: “no created nature is a sufficient principle of an act meritorious of eternal life, unless there is added a supernatural gift, which we call grace” (St. Thomas, ST I-II q.114 a.2). And as St. Augustine himself says, “our merits are gifts effected through grace; when God rewards, He crowns His gifts, even the merits He has given…” (Letter 194, to St. Sixtus). We shall encounter this again later in On Free Choice of the Free Will, but I thought it worth mentioning here, too.
Trent has something to say about this as well.
CANON XXXII.-If any one saith, that the good works of one that is justified are in such manner the gifts of God, as that they are not also the good merits of him that is justified; or, that the said justified, by the good works which he performs through the grace of God and the merit of Jesus Christ, whose living member he is, does not truly merit increase of grace, eternal life, and the attainment of that eternal life,-if so be, however, that he depart in grace,-and also an increase of glory; let him be anathema. [Canons on Justification]
Protestants deny this truth, and this is another way in which they contradict St. Augustine.