Objection 1. It would seem that not all the moral precepts of the Old Law are reducible to the ten precepts of the decalogue. For the first and principal precepts of the Law are, "Thou shalt love the Lord thy God," and "Thou shalt love thy neighbor," as stated in Matthew 22:37-39. But these two are not contained in the precepts of the decalogue. Therefore not all the moral precepts are contained in the precepts of the decalogue.
Objection 2. Further, the moral precepts are not reducible to the ceremonial precepts, but rather vice versa. But among the precepts of the decalogue, one is ceremonial, viz. "Remember that thou keep holy the Sabbath-day." Therefore the moral precepts are not reducible to all the precepts of the decalogue.
Objection 3. Further, the moral precepts are about all the acts of virtue. But among the precepts of the decalogue are only such as regard acts of justice; as may be seen by going through them all. Therefore the precepts of the decalogue do not include all the moral precepts.
On the contrary, The gloss on Matthew 5:11: "Blessed are ye when they shall revile you," etc. says that "Moses, after propounding the ten precepts, set them out in detail." Therefore all the precepts of the Law are so many parts of the precepts of the decalogue.
I answer that, The precepts of the decalogue differ from the other precepts of the Law, in the fact that God Himself is said to have given the precepts of the decalogue; whereas He gave the other precepts to the people through Moses. Wherefore the decalogue includes those precepts the knowledge of which man has immediately from God. Such are those which with but slight reflection can be gathered at once from the first general principles: and those also which become known to man immediately through divinely infused faith. Consequently two kinds of precepts are not reckoned among the precepts of the decalogue: viz. first general principles, for they need no further promulgation after being once imprinted on the natural reason to which they are self-evident; as, for instance, that one should do evil to no man, and other similar principles: and again those which the careful reflection of wise men shows to be in accord with reason; since the people receive these principles from God, through being taught by wise men. Nevertheless both kinds of precepts are contained in the precepts of the decalogue; yet in different ways. For the first general principles are contained in them, as principles in their proximate conclusions; while those which are known through wise men are contained, conversely, as conclusions in their principles.
Reply to Objection 1. Those two principles are the first general principles of the natural law, and are self-evident to human reason, either through nature or through faith. Wherefore all the precepts of the decalogue are referred to these, as conclusions to general principles.
Reply to Objection 2. The precept of the Sabbath observance is moral in one respect, in so far as it commands man to give some time to the things of God, according to Psalm 45:11: "Be still and see that I am God." In this respect it is placed among the precepts of the decalogue: but not as to the fixing of the time, in which respect it is a ceremonial precept.
Reply to Objection 3. The notion of duty is not so patent in the other virtues as it is in justice. Hence the precepts about the acts of the other virtues are not so well known to the people as are the precepts about acts of justice. Wherefore the acts of justice especially come under the precepts of the decalogue, which are the primary elements of the Law.
Source: Summa Theologica (New Advent)