Objection 1. It would seem that the Old Law was not from God. For it is written (Deuteronomy 32:4): "The works of God are perfect." But the Law was imperfect, as stated above (Article 1). Therefore the Old Law was not from God.
Objection 2. Further, it is written (Ecclesiastes 3:14): "I have learned that all the works which God hath made continue for ever." But the Old Law does not continue for ever: since the Apostle says (Hebrews 7:18): "There is indeed a setting aside of the former commandment, because of the weakness and unprofitableness thereof." Therefore the Old Law was not from God.
Objection 3. Further, a wise lawgiver should remove, not only evil, but also the occasions of evil. But the Old Law was an occasion of sin, as stated above (1, ad 2). Therefore the giving of such a law does not pertain to God, to Whom "none is like among the lawgivers" (Job 36:22).
Objection 4. Further, it is written (1 Timothy 2:4) that God "will have all men to be saved." But the Old Law did not suffice to save man, as stated above (Article 1). Therefore the giving of such a law did not appertain to God. Therefore the Old Law was not from God.
On the contrary, Our Lord said (Matthew 15:6) while speaking to the Jews, to whom the Law was given: "You have made void the commandment of God for your tradition." And shortly before (verse 4) He had said: "Honor thy father and mother," which is contained expressly in the Old Law (Exodus 20:12; Deuteronomy 5:16). Therefore the Old Law was from God.
I answer that, The Old Law was given by the good God, Who is the Father of Our Lord Jesus Christ. For the Old Law ordained men to Christ in two ways. First by bearing witness to Christ; wherefore He Himself says (Luke 24:44): "All things must needs be fulfilled, which are written in the law... and in the prophets, and in the psalms, concerning Me": and (John 5:46): "If you did believe Moses, you would perhaps believe Me also; for he wrote of Me." Secondly, as a kind of disposition, since by withdrawing men from idolatrous worship, it enclosed [concludebat] them in the worship of one God, by Whom the human race was to be saved through Christ. Wherefore the Apostle says (Galatians 3:23): "Before the faith came, we were kept under the law shut up [conclusi], unto that faith which was to be revealed." Now it is evident that the same thing it is, which gives a disposition to the end, and which brings to the end; and when I say "the same," I mean that it does so either by itself or through its subjects. For the devil would not make a law whereby men would be led to Christ, Who was to cast him out, according to Matthew 12:26: "If Satan cast out Satan, his kingdom is divided" [Vulgate: 'he is divided against himself']. Therefore the Old Law was given by the same God, from Whom came salvation to man, through the grace of Christ.
Reply to Objection 1. Nothing prevents a thing being not perfect simply, and yet perfect in respect of time: thus a boy is said to be perfect, not simply, but with regard to the condition of time. So, too, precepts that are given to children are perfect in comparison with the condition of those to whom they are given, although they are not perfect simply. Hence the Apostle says (Galatians 3:24): "The law was our pedagogue in Christ."
Reply to Objection 2. Those works of God endure for ever which God so made that they would endure for ever; and these are His perfect works. But the Old Law was set aside when there came the perfection of grace; not as though it were evil, but as being weak and useless for this time; because, as the Apostle goes on to say, "the law brought nothing to perfection": hence he says (Galatians 3:25): "After the faith is come, we are no longer under a pedagogue."
Reply to Objection 3. As stated above (Question 79, Article 4), God sometimes permits certain ones to fall into sin, that they may thereby be humbled. So also did He wish to give such a law as men by their own forces could not fulfill, so that, while presuming on their own powers, they might find themselves to be sinners, and being humbled might have recourse to the help of grace.
Reply to Objection 4. Although the Old Law did not suffice to save man, yet another help from God besides the Law was available for man, viz. faith in the Mediator, by which the fathers of old were justified even as we were. Accordingly God did not fail man by giving him insufficient aids to salvation.
Source: Summa Theologica (New Advent)