Objection 1. It would seem that the Old Law was not good. For it is written (Ezekiel 20:25): "I gave them statutes that were not good, and judgments in which they shall not live." But a law is not said to be good except on account of the goodness of the precepts that it contains. Therefore the Old Law was not good.
Objection 2. Further, it belongs to the goodness of a law that it conduce to the common welfare, as Isidore says (Etym. v, 3). But the Old Law was not salutary; rather was it deadly and hurtful. For the Apostle says (Romans 7:8, seqq.): "Without the law sin was dead. And I lived some time without the law. But when the commandment came sin revived; and I died." Again he says (Romans 5:20): "Law entered in that sin might abound." Therefore the Old Law was not good.
Objection 3. Further, it belongs to the goodness of the law that it should be possible to obey it, both according to nature, and according to human custom. But such the Old Law was not: since Peter said (Acts 15:10): "Why tempt you (God) to put a yoke on the necks of the disciples, which neither our fathers nor we have been able to bear?" Therefore it seems that the Old Law was not good.
On the contrary, The Apostle says (Romans 7:12): "Wherefore the law indeed is holy, and the commandment holy, and just, and good."
I answer that, Without any doubt, the Old Law was good. For just as a doctrine is shown to be good by the fact that it accords with right reason, so is a law proved to be good if it accords with reason. Now the Old Law was in accordance with reason. Because it repressed concupiscence which is in conflict with reason, as evidenced by the commandment, "Thou shalt not covet thy neighbor's goods" (Exodus 20:17). Moreover the same law forbade all kinds of sin; and these too are contrary to reason. Consequently it is evident that it was a good law. The Apostle argues in the same way (Romans 7): "I am delighted," says he (verse 22), "with the law of God, according to the inward man": and again (verse 16): "I consent to the law, that is good."
But it must be noted that the good has various degrees, as Dionysius states (Div. Nom. iv): for there is a perfect good, and an imperfect good. In things ordained to an end, there is perfect goodness when a thing is such that it is sufficient in itself to conduce to the end: while there is imperfect goodness when a thing is of some assistance in attaining the end, but is not sufficient for the realization thereof. Thus a medicine is perfectly good, if it gives health to a man; but it is imperfect, if it helps to cure him, without being able to bring him back to health. Again it must be observed that the end of human law is different from the end of Divine law. For the end of human law is the temporal tranquillity of the state, which end law effects by directing external actions, as regards those evils which might disturb the peaceful condition of the state. On the other hand, the end of the Divine law is to bring man to that end which is everlasting happiness; which end is hindered by any sin, not only of external, but also of internal action. Consequently that which suffices for the perfection of human law, viz. the prohibition and punishment of sin, does not suffice for the perfection of the Divine law: but it is requisite that it should make man altogether fit to partake of everlasting happiness. Now this cannot be done save by the grace of the Holy Ghost, whereby "charity" which fulfilleth the law... "is spread abroad in our hearts" (Romans 5:5): since "the grace of God is life everlasting" (Romans 6:23). But the Old Law could not confer this grace, for this was reserved to Christ; because, as it is written (John 1:17), the law was given "by Moses, grace and truth came by Jesus Christ." Consequently the Old Law was good indeed, but imperfect, according to Hebrews 7:19: "The law brought nothing to perfection."
Reply to Objection 1. The Lord refers there to the ceremonial precepts; which are said not to be good, because they did not confer grace unto the remission of sins, although by fulfilling these precepts man confessed himself a sinner. Hence it is said pointedly, "and judgments in which they shall not live"; i.e. whereby they are unable to obtain life; and so the text goes on: "And I polluted them," i.e. showed them to be polluted, "in their own gifts, when they offered all that opened the womb, for their offenses."
Reply to Objection 2. The law is said to have been deadly, as being not the cause, but the occasion of death, on account of its imperfection: in so far as it did not confer grace enabling man to fulfil what is prescribed, and to avoid what it forbade. Hence this occasion was not given to men, but taken by them. Wherefore the Apostle says (Romans 5:11): "Sin, taking occasion by the commandment, seduced me, and by it killed me." In the same sense when it is said that "the law entered in that sin might abound," the conjunction "that" must be taken as consecutive and not final: in so far as men, taking occasion from the law, sinned all the more, both because a sin became more grievous after law had forbidden it, and because concupiscence increased, since we desire a thing the more from its being forbidden.
Reply to Objection 3. The yoke of the law could not be borne without the help of grace, which the law did not confer: for it is written (Romans 9:16): "It is not him that willeth, nor of him that runneth," viz. that he wills and runs in the commandments of God, "but of God that showeth mercy." Wherefore it is written (Psalm 118:32): "I have run the way of Thy commandments, when Thou didst enlarge my heart," i.e. by giving me grace and charity.
Source: Summa Theologica (New Advent)