Objection 1. It seems that the second precept of the decalogue is unfittingly expressed. For this precept, "Thou shalt not take the name of thy God in vain" is thus explained by a gloss on Exodus 20:7: "Thou shalt not deem the Son of God to be a creature," so that it forbids an error against faith. Again, a gloss on the words of Deuteronomy 5:11, "Thou shalt not take the name of... thy God in vain, " adds, i.e. "by giving the name of God to wood or stone," as though they forbade a false confession of faith, which, like error, is an act of unbelief. Now unbelief precedes superstition, as faith precedes religion. Therefore this precept should have preceded the first, whereby superstition is forbidden.
Objection 2. Further, the name of God is taken for many purposes --for instance, those of praise, of working miracles, and generally speaking in conjunction with all we say or do, according to Colossians 3:17, "All whatsoever you do in word or in work... do ye in the name of the Lord." Therefore the precept forbidding the taking of God's name in vain seems to be more universal than the precept forbidding superstition, and thus should have preceded it.
Objection 3. Further, a gloss on Exodus 20:7 expounds the precept, "Thou shalt not take the name of... thy God in vain," namely, by swearing to nothing. Hence this precept would seem to forbid useless swearing, that is to say, swearing without judgment. But false swearing, which is without truth, and unjust swearing, which is without justice, are much more grievous. Therefore this precept should rather have forbidden them.
Objection 4. Further, blasphemy or any word or deed that is an insult to God is much more grievous than perjury. Therefore blasphemy and other like sins should rather have been forbidden by this precept.
Objection 5. Further, God's names are many. Therefore it should not have been said indefinitely: "Thou shalt not take the name of... thy God in vain."
On the contrary, stands the authority of Scripture.
I answer that, In one who is being instructed in virtue it is necessary to remove obstacles to true religion before establishing him in true religion. Now a thing is opposed to true religion in two ways. First, by excess, when, to wit, that which belongs to religion is given to others than to whom it is due, and this pertains to superstition. Secondly, by lack, as it were, of reverence, when, to wit, God is contemned, and this pertains to the vice of irreligion, as stated above (97, in the preamble, and in the Article that follows). Now superstition hinders religion by preventing man from acknowledging God so as to worship Him: and when a man's mind is engrossed in some undue worship, he cannot at the same time give due worship to God, according to Isaiah 28:20, "The bed is straitened, so that one must fall out," i.e. either the true God or a false god must fall out from man's heart, "and a short covering cannot cover both." On the other hand, irreligion hinders religion by preventing man from honoring God after he has acknowledged Him. Now one must first of all acknowledge God with a view to worship, before honoring Him we have acknowledged.
For this reason the precept forbidding superstition is placed before the second precept, which forbids perjury that pertains to irreligion.
Reply to Objection 1. These expositions are mystical. The literal explanation is that which is given Deuteronomy 5:11: "Thou shalt not take the name of... thy God in vain," namely, "by swearing on that which is not [Vulgate: 'for he shall not be unpunished that taketh His name upon a vain thing']."
Reply to Objection 2. This precept does not forbid all taking of the name of God, but properly the taking of God's name in confirmation of a man's word by way of an oath, because men are wont to take God's name more frequently in this way. Nevertheless we may understand that in consequence all inordinate taking of the Divine name is forbidden by this precept: and it is in this sense that we are to take the explanation quoted in the First Objection.
Reply to Objection 3. To swear to nothing means to swear to that which is not. This pertains to false swearing, which is chiefly called perjury, as stated above (98, 1, ad 3). For when a man swears to that which is false, his swearing is vain in itself, since it is not supported by the truth. on the other hand, when a man swears without judgment, through levity, if he swear to the truth, there is no vanity on the part of the oath itself, but only on the part of the swearer.
Reply to Objection 4. Just as when we instruct a man in some science, we begin by putting before him certain general maxims, even so the Law, which forms man to virtue by instructing him in the precepts of the decalogue, which are the first of all precepts, gave expression, by prohibition or by command, to those things which are of most common occurrence in the course of human life. Hence the precepts of the decalogue include the prohibition of perjury, which is of more frequent occurrence than blasphemy, since man does not fall so often into the latter sin.
Reply to Objection 5. Reverence is due to the Divine names on the part of the thing signified, which is one, and not on the part of the signifying words, which are many. Hence it is expressed in the singular: "Thou shalt not take the name of... thy God in vain": since it matters not in which of God's names perjury is committed.
Source: Summa Theologica (New Advent)