Objection 1. It would seem that the moral precepts of the Old Law justified man. Because the Apostle says (Romans 2:13): "For not the hearers of the Law are justified before God, but the doers of the Law shall be justified." But the doers of the Law are those who fulfil the precepts of the Law. Therefore the fulfilling of the precepts of the Law was a cause of justification.
Objection 2. Further, it is written (Leviticus 18:5): "Keep My laws and My judgments, which if a man do, he shall live in them." But the spiritual life of man is through justice. Therefore the fulfilling of the precepts of the Law was a cause of justification.
Objection 3. Further, the Divine law is more efficacious than human law. But human law justifies man; since there is a kind of justice consisting in fulfilling the precepts of law. Therefore the precepts of the Law justified man.
On the contrary, The Apostle says (2 Corinthians 3:6): "The letter killeth": which, according to Augustine (De Spir. et Lit. xiv), refers even to the moral precepts. Therefore the moral precepts did not cause justice.
I answer that, Just as "healthy" is said properly and first of that which is possessed of health, and secondarily of that which is a sign or a safeguard of health; so justification means first and properly the causing of justice; while secondarily and improperly, as it were, it may denote a sign of justice or a disposition thereto. If justice be taken in the last two ways, it is evident that it was conferred by the precepts of the Law; in so far, to wit, as they disposed men to the justifying grace of Christ, which they also signified, because as Augustine says (Contra Faust. xxii, 24), "even the life of that people foretold and foreshadowed Christ."
But if we speak of justification properly so called, then we must notice that it can be considered as in the habit or as in the act: so that accordingly justification may be taken in two ways. First, according as man is made just, by becoming possessed of the habit of justice: secondly, according as he does works of justice, so that in this sense justification is nothing else than the execution of justice. Now justice, like the other virtues, may denote either the acquired or the infused virtue, as is clear from what has been stated (63, 4). The acquired virtue is caused by works; but the infused virtue is caused by God Himself through His grace. The latter is true justice, of which we are speaking now, and in this respect of which a man is said to be just before God, according to Romans 4:2: "If Abraham were justified by works, he hath whereof to glory, but not before God." Hence this justice could not be caused by moral precepts, which are about human actions: wherefore the moral precepts could not justify man by causing justice.
If, on the other hand, by justification we understand the execution of justice, thus all the precepts of the Law justified man, but in various ways. Because the ceremonial precepts taken as a whole contained something just in itself, in so far as they aimed at offering worship to God; whereas taken individually they contained that which is just, not in itself, but by being a determination of the Divine law. Hence it is said of these precepts that they did not justify man save through the devotion and obedience of those who complied with them. On the other hand the moral and judicial precepts, either in general or also in particular, contained that which is just in itself: but the moral precepts contained that which is just in itself according to that "general justice" which is "every virtue" according to Ethic. v, 1: whereas the judicial precepts belonged to "special justice," which is about contracts connected with the human mode of life, between one man and another.
Reply to Objection 1. The Apostle takes justification for the execution of justice.
Reply to Objection 2. The man who fulfilled the precepts of the Law is said to live in them, because he did not incur the penalty of death, which the Law inflicted on its transgressors: in this sense the Apostle quotes this passage (Galatians 3:12).
Reply to Objection 3. The precepts of human law justify man by acquired justice: it is not about this that we are inquiring now, but only about that justice which is before God.
Source: Summa Theologica (New Advent)