Objection 1. It would seem that the precepts of the decalogue are unsuitably distinguished from one another. For worship is a virtue distinct from faith. Now the precepts are about acts of virtue. But that which is said at the beginning of the decalogue, "Thou shalt not have strange gods before Me," belongs to faith: and that which is added, "Thou shalt not make... any graven thing," etc. belongs to worship. Therefore these are not one precept, as Augustine asserts (Qq. in Exod. qu. lxxi), but two.
Objection 2. Further, the affirmative precepts in the Law are distinct from the negative precepts; e.g. "Honor thy father and thy mother," and, "Thou shalt not kill." But this, "I am the Lord thy God," is affirmative: and that which follows, "Thou shalt not have strange gods before Me," is negative. Therefore these are two precepts, and do not, as Augustine says (Qq. in Exod. qu. lxxi), make one.
Objection 3. Further, the Apostle says (Romans 7:7): "I had not known concupiscence, if the Law did not say: 'Thou shalt not covet.'" Hence it seems that this precept, "Thou shalt not covet," is one precept; and, therefore, should not be divided into two.
On the contrary, stands the authority of Augustine who, in commenting on Exodus (Qq. in Exod. qu. lxxi) distinguishes three precepts as referring to God, and seven as referring to our neighbor.
I answer that, The precepts of the decalogue are differently divided by different authorities. For Hesychius commenting on Leviticus 26:26, "Ten women shall bake your bread in one oven," says that the precept of the Sabbath-day observance is not one of the ten precepts, because its observance, in the letter, is not binding for all time. But he distinguishes four precepts pertaining to God, the first being, "I am the Lord thy God"; the second, "Thou shalt not have strange gods before Me," (thus also Jerome distinguishes these two precepts, in his commentary on Hosea 10:10, "On thy" [Vulgate: "their"] "two iniquities"); the third precept according to him is, "Thou shalt not make to thyself any graven thing"; and the fourth, "Thou shalt not take the name of the Lord thy God in vain." He states that there are six precepts pertaining to our neighbor; the first, "Honor thy father and thy mother"; the second, "Thou shalt not kill"; the third, "Thou shalt not commit adultery"; the fourth, "Thou shalt not steal"; the fifth, "Thou shalt not bear false witness"; the sixth, "Thou shalt not covet."
But, in the first place, it seems unbecoming for the precept of the Sabbath-day observance to be put among the precepts of the decalogue, if it nowise belonged to the decalogue. Secondly, because, since it is written (Matthew 6:24), "No man can serve two masters," the two statements, "I am the Lord thy God," and, "Thou shalt not have strange gods before Me" seem to be of the same nature and to form one precept. Hence Origen (Hom. viii in Exod.) who also distinguishes four precepts as referring to God, unites these two under one precept; and reckons in the second place, "Thou shalt not make... any graven thing"; as third, "Thou shalt not take the name of the Lord thy God in vain"; and as fourth, "Remember that thou keep holy the Sabbath-day." The other six he reckons in the same way as Hesychius.
Since, however, the making of graven things or the likeness of anything is not forbidden except as to the point of their being worshipped as gods--for God commanded an image of the Seraphim [Vulgate: Cherubim] to be made and placed in the tabernacle, as related in Exodus 25:18--Augustine more fittingly unites these two, "Thou shalt not have strange gods before Me," and, "Thou shalt not make... any graven thing," into one precept. Likewise to covet another's wife, for the purpose of carnal knowledge, belongs to the concupiscence of the flesh; whereas, to covet other things, which are desired for the purpose of possession, belongs to the concupiscence of the eyes; wherefore Augustine reckons as distinct precepts, that which forbids the coveting of another's goods, and that which prohibits the coveting of another's wife. Thus he distinguishes three precepts as referring to God, and seven as referring to our neighbor. And this is better.
Reply to Objection 1. Worship is merely a declaration of faith: wherefore the precepts about worship should not be reckoned as distinct from those about faith. Nevertheless precepts should be given about worship rather than about faith, because the precept about faith is presupposed to the precepts of the decalogue, as is also the precept of charity. For just as the first general principles of the natural law are self-evident to a subject having natural reason, and need no promulgation; so also to believe in God is a first and self-evident principle to a subject possessed of faith: "for he that cometh to God, must believe that He is" (Hebrews 11:6). Hence it needs no other promulgation that the infusion of faith.
Reply to Objection 2. The affirmative precepts are distinct from the negative, when one is not comprised in the other: thus that man should honor his parents does not include that he should not kill another man; nor does the latter include the former. But when an affirmative precept is included in a negative, or vice versa, we do not find that two distinct precepts are given: thus there is not one precept saying that "Thou shalt not steal," and another binding one to keep another's property intact, or to give it back to its owner. In the same way there are not different precepts about believing in God, and about not believing in strange gods.
Reply to Objection 3. All covetousness has one common ratio: and therefore the Apostle speaks of the commandment about covetousness as though it were one. But because there are various special kinds of covetousness, therefore Augustine distinguishes different prohibitions against coveting: for covetousness differs specifically in respect of the diversity of actions or things coveted, as the Philosopher says (Ethic. x, 5).
Source: Summa Theologica (New Advent)