Objection 1. It would seem that, in the Old Law, there should have been given precepts of faith. Because a precept is about something due and necessary. Now it is most necessary for man that he should believe, according to Hebrews 11:6, "Without faith it is impossible to please God." Therefore there was very great need for precepts of faith to be given.
Objection 2. Further, the New Testament is contained in the Old, as the reality in the figure, as stated above (I-II, 107, 3). Now the New Testament contains explicit precepts of faith, for instance John 14:1: "You believe in God; believe also in Me." Therefore it seems that some precepts of faith ought to have been given in the Old Law also.
Objection 3. Further, to prescribe the act of a virtue comes to the same as to forbid the opposite vices. Now the Old Law contained many precepts forbidding unbelief: thus (Exodus 20:3): "Thou shalt not have strange gods before Me," and (Deuteronomy 13:1-3) they were forbidden to hear the words of the prophet or dreamer who might wish to turn them away from their faith in God. Therefore precepts of faith should have been given in the Old Law also.
Objection 4. Further, confession is an act of faith, as stated above (Question 3, Article 1). Now the Old Law contained precepts about the confession and the promulgation of faith: for they were commanded (Exodus 12:27) that, when their children should ask them, they should tell them the meaning of the paschal observance, and (Deuteronomy 13:9) they were commanded to slay anyone who disseminated doctrine contrary to faith. Therefore the Old Law should have contained precepts of faith.
Objection 5. Further, all the books of the Old Testament are contained in the Old Law; wherefore Our Lord said (John 15:25) that it was written in the Law: "They have hated Me without cause," although this is found written in Psalm 34 and 68. Now it is written (Sirach 2:8): "Ye that fear the Lord, believe Him." Therefore the Old Law should have contained precepts of faith.
On the contrary, The Apostle (Romans 3:27) calls the Old Law the "law of works" which he contrasts with the "law of faith." Therefore the Old Law ought not to have contained precepts of faith.
I answer that, A master does not impose laws on others than his subjects; wherefore the precepts of a law presuppose that everyone who receives the law is subject to the giver of the law. Now the primary subjection of man to God is by faith, according to Hebrews 11:6: "He that cometh to God, must believe that He is." Hence faith is presupposed to the precepts of the Law: for which reason (Exodus 20:2) that which is of faith, is set down before the legal precepts, in the words, "I am the Lord thy God, Who brought thee out of the land of Egypt," and, likewise (Deuteronomy 6:4), the words, "Hear, O Israel, the Lord thy [Vulgate: 'our'] God is one," precede the recording of the precepts.
Since, however, faith contains many things subordinate to the faith whereby we believe that God is, which is the first and chief of all articles of faith, as stated above (1, 1,7), it follows that, if we presuppose faith in God, whereby man's mind is subjected to Him, it is possible for precepts to be given about other articles of faith. Thus Augustine expounding the words: "This is My commandment" (John 15:12) says (Tract. lxxxiii in Joan.) that we have received many precepts of faith. In the Old Law, however, the secret things of faith were not to be set before the people, wherefore, presupposing their faith in one God, no other precepts of faith were given in the Old Law.
Reply to Objection 1. Faith is necessary as being the principle of spiritual life, wherefore it is presupposed before the receiving of the Law.
Reply to Objection 2. Even then Our Lord both presupposed something of faith, namely belief in one God, when He said: "You believe in God," and commanded something, namely, belief in the Incarnation whereby one Person is God and man. This explanation of faith belongs to the faith of the New Testament, wherefore He added: "Believe also in Me."
Reply to Objection 3. The prohibitive precepts regard sins, which corrupt virtue. Now virtue is corrupted by any particular defect, as stated above (I-II, 18, 4, ad 3; I-II, 19, 6, ad 1, 7, ad 3). Therefore faith in one God being presupposed, prohibitive precepts had to be given in the Old Law, so that men might be warned off those particular defects whereby their faith might be corrupted.
Reply to Objection 4. Confession of faith and the teaching thereof also presuppose man's submission to God by faith: so that the Old Law could contain precepts relating to the confession and teaching of faith, rather than to faith itself.
Reply to Objection 5. In this passage again that faith is presupposed whereby we believe that God is; hence it begins, "Ye that fear the Lord," which is not possible without faith. The words which follow--"believe Him"--must be referred to certain special articles of faith, chiefly to those things which God promises to them that obey Him, wherefore the passage concludes--"and your reward shall not be made void."
Source: Summa Theologica (New Advent)