Objection 1. It seems that the first precept of the decalogue is unfittingly expressed. For man is more bound to God than to his father in the flesh, according to Hebrews 12:9, "How much more shall we [Vulgate: 'shall we not much more'] obey the Father of spirits and live?" Now the precept of piety, whereby man honors his father, is expressed affirmatively in these words: "Honor thy father and thy mother." Much more, therefore, should the first precept of religion, whereby all honor God, be expressed affirmatively, especially as affirmation is naturally prior to negation.
Objection 2. Further, the first precept of the decalogue pertains to religion, as stated above (Article 1). Now religion, since it is one virtue, has one act. Yet in the first precept three acts are forbidden: since we read first: "Thou shalt not have strange gods before Me"; secondly, "Thou shalt not make to thyself any graven thing"; and thirdly, "Thou shalt not adore them nor serve them." Therefore the first precept is unfittingly expressed.
Objection 3. Further, Augustine says (De decem chord. ix) that "the first precept forbids the sin of superstition." But there are many wicked superstitions besides idolatry, as stated above (Question 92, Article 2). Therefore it was insufficient to forbid idolatry alone.
On the contrary, stands the authority of Scripture.
I answer that, It pertains to law to make men good, wherefore it behooved the precepts of the Law to be set in order according to the order of generation, the order, to wit, of man's becoming good. Now two things must be observed in the order of generation. The first is that the first part is the first thing to be established; thus in the generation of an animal the first thing to be formed is the heart, and in building a home the first thing to be set up is the foundation: and in the goodness of the soul the first part is goodness of the will, the result of which is that a man makes good use of every other goodness. Now the goodness of the will depends on its object, which is its end. Wherefore since man was to be directed to virtue by means of the Law, the first thing necessary was, as it were, to lay the foundation of religion, whereby man is duly directed to God, Who is the last end of man's will.
The second thing to be observed in the order of generation is that in the first place contraries and obstacles have to be removed. Thus the farmer first purifies the soil, and afterwards sows his seed, according to Jeremiah 4:3, "Break up anew your fallow ground, and sow not upon thorns." Hence it behooved man, first of all to be instructed in religion, so as to remove the obstacles to true religion. Now the chief obstacle to religion is for man to adhere to a false god, according to Matthew 6:24, "You cannot serve God and mammon." Therefore in the first precept of the Law the worship of false gods is excluded.
Reply to Objection 1. In point of fact there is one affirmative precept about religion, namely: "Remember that thou keep holy the Sabbath Day." Still the negative precepts had to be given first, so that by their means the obstacles to religion might be removed. For though affirmation naturally precedes negation, yet in the process of generation, negation, whereby obstacles are removed, comes first, as stated in the Article. Especially is this true in matters concerning God, where negation is preferable to affirmation, on account of our insufficiency, as Dionysius observes (Coel. Hier. ii).
Reply to Objection 2. People worshiped strange gods in two ways. For some served certain creatures as gods without having recourse to images. Hence Varro says that for a long time the ancient Romans worshiped gods without using images: and this worship is first forbidden by the words, "Thou shalt not have strange gods." Among others the worship of false gods was observed by using certain images: and so the very making of images was fittingly forbidden by the words, "Thou shalt not make to thyself any graven thing," as also the worship of those same images, by the words, "Thou shalt not adore them," etc.
Reply to Objection 3. All other kinds of superstition proceed from some compact, tacit or explicit, with the demons; hence all are understood to be forbidden by the words, "Thou shalt not have strange gods."
Source: Summa Theologica (New Advent)