The Catechism of St. Thomas Aquinas

SECOND COMMANDMENT: “Thou Shalt Not Take the Name of the Lord Thy God in Vain.”

This is the Second Commandment of the law. Just as there is but one God whom we must worship, so there is only one God whom we should reverence in a special manner. This, first of all, has reference to the name of God. “Thou shalt not take the name of the Lord thy God in vain.”[1]


“In vain” has a threefold meaning. Sometimes it is said of that which is false: “They have spoken vain things every one to his neighbor.”[2] One, therefore, takes the name of God in vain when one uses it to confirm that which is not true: “Love not a false oath.”[3] “Thou shalt not live because thou hast spoken a lie in the name of the Lord.”[4] Any one so doing does injury to God, to himself, and to all men.

It is an insult to God because, when you swear by God, it is nothing other than to call Him to witness; and when you swear falsely, you either believe God to be ignorant of the truth and thus place ignorance in God, whereas “all things are naked and open to His eyes,”[5] or you think that God loves a lie, whereas He hates it: “Thou wilt destroy all that speak a lie.”[6] Or, again, you detract from His power, as if He were not able to punish a lie.

Likewise, such a one does an injury to himself, for he binds himself to the judgment of God. It is the same thing to say, “By God this is so,” as to say, “May God punish me if it is not so!”

He, finally, does an injury to other men. For there can be no lasting society unless men believe one another. Matters that are doubtful may be confirmed by oaths: “An oath in confirmation puts an end to all controversy.”[7] Therefore, he who violates this precept does injury to God, is cruel to himself, and harmful to other men.

Sometimes “vain” signifies useless: “The Lord knoweth the thoughts of men, that they are vain.”[8] God’s name, therefore, is taken in vain when it is used to confirm vain things.

In the Old Law it was forbidden to swear falsely: “Thou shalt not take the name of the Lord thy God in vain.”[9] And Christ forbade the taking of oaths except in case of necessity: “You have heard that it was said to them of old: Thou shalt not forswear thyself. . . . But I say to you not to swear at all.”[10] And the reason for this is that in no part of our body are we so weak as in the tongue, for “the tongue no man can tame.”[11] And thus even in light matter one can perjure himself. “Let your speech be: Yea, yea; No, no. But I say to you not to swear at all.”[12]

Note well that an oath is like medicine, which is never taken continually but only in times of necessity. Hence, the Lord adds: “And that which is over and above these is evil.”[13] “Let not the mouth be accustomed to swearing, for in it there are many falls. And let not the name of God be usual in thy mouth, and meddle not with the names of saints. For thou shalt not escape free from them.”[14]

Sometimes “in vain” means sin or injustice: “O ye sons of men, how long will you be dull of heart? Why do you love vanity?”[15] Therefore, he who swears to commit a sin, takes the name of his God in vain. Justice consists in doing good and avoiding evil. Therefore, if you take an oath to steal or commit some crime of this sort, you sin against justice. And although you must not keep this oath, you are still guilty of perjury. Herod did this against John.[16] It is likewise against justice when one swears not to do some good act, as not to enter a church or a religious community. And although this oath, too, is not binding, yet, despite this, the person himself is a perjuror.


One cannot, therefore, swear to a falsehood, or without good reason, or in any way against justice: “And thou shalt swear: As the Lord liveth, in truth, and in judgment and in justice.”[17]

Sometimes “vain” also means foolish: “All men are vain, in whom there is not the knowledge of God.”[18] Accordingly, he who takes the name of God foolishly, by blasphemy, takes the name of God in vain: “And he that blasphemeth the name of the Lord, dying let him die.”[19]


“Thou shalt not take the name of the Lord thy God in vain.” However, the name of God may be taken for six purposes. First, to confirm something that is said, as in an oath. In this we show God alone is the first Truth, and also we show due reverence to God. For this reason it was commanded in the Old Law that one must not swear except by God.[20] They who swore otherwise violated this order: “By the name of strange gods you shall not swear.”[21] Although at times one swears by creatures, nevertheless, it must be known that such is the same as swearing by God. When you swear by your soul or your head, it is as if you bind yourself to be punished by God. Thus: “But I call God to witness upon my soul.”[22] And when you swear by the Gospel, you swear by God who gave the Gospel. But they sin who swear either by God or by the Gospel for any trivial reason.

The second purpose is that of sanctification. Thus, Baptism sanctifies, for as St. Paul says: “But you are washed, but you are sanctified, but you are justified in the name of our Lord Jesus Christ, and the Spirit of God.”[23] Baptism, however, does not have power except through the invocation of the Trinity: “But Thou, O Lord, art among us, and Thy name is called upon by us.”[24]

The third purpose is the expulsion of our adversary; hence, before Baptism we renounce the devil: “Only let Thy name be called upon us; take away our reproach.[25] Wherefore, if one return to his sins, the name of God has been taken in vain.

Fourthly, God’s name is taken in order to confess it: “How then shall they call on Him, in whom they have not believed?”[26] And again: “Whosoever shall call upon the name of the Lord, shall be saved.”[27] First of all, we confess by word of mouth that we may show forth the glory of God: “And every one that calleth upon My name, I have created him for My glory.”[28] Accordingly, if one says anything against the glory of God, he takes the name of God in vain. Secondly, we confess God’s name by our works, when our very actions show forth God’s glory: “That they may see your good works, and may glorify your Father who is in heaven.”[29] “Through you the name of God is blasphemed among the Gentiles.”[30]

Fifthly it is taken for our defense: “The name of the Lord is a strong tower; the just runneth to it and shall be exalted.”[31] “In My name they shall cast out devils.”[32] “There is no other name under heaven given to men, whereby we must be saved.[33]

Lastly, it is taken in order to make our works complete. Thus says the Apostle: “All whatsoever you do in word or work, do all in the name of the Lord Jesus Christ.”[34] The reason is because “our help is in the name of the Lord.”[35] Sometimes it happens that one begins a work imprudently by starting with a vow, for instance, and then not completing either the work or the vow. And this again is taking God’s name in vain. “If thou hast vowed anything to God, defer not to pay it.”[36] “Vow and pay to the Lord your God; all ye that are round about Him bring presents.”[37] “For an unfaithful and foolish promise displeaseth Him.”[38]

(For “Questions for Discussion” see Chapter 6.)


1. “He who requires that honor be paid Him, also demands that we speak of Him with reverence, and He forbids the contrary. . . . There are those who are so blinded by darkness of error as not to fear to blaspheme His name, whom the Angels glorify. Men are not deterred by this Commandment from shamelessly and daringly outraging His divine majesty every day, or rather every hour and moment of the day. Who does not know that every assertion is accompanied with an oath and teems with curses and imprecations? To such lengths has this impiety been carried that one scarcely buys or sells, or transacts ordinary business of any sort, without having recourse to swearing, and who, even in matters the most unimportant and trivial, does not profane the most holy name of God thousands of times” (“Roman Catechism,” “Second Commandment,” 2). See also teaching of St. Thomas in “Summa Theol.,” II-II, Q. lxxxix, art. 3, 5, 6.

2. Ps. xi. 3.

3. Zach, viii. 17.

4. “Ibid.,” xiii. 3.

5. Heb., iv. 13.

6. Ps. v. 7.

7. Heb., vi. 16.

8. Ps. xciii. 11.

9. Deut., v. 11.

10. Matt., v. 33-34.

11. James, iii. 8.

12. Matt., v. 34, 37. “It cannot be stated that these words condemn oaths universally and under all circumstances, since the Apostles and Our Lord Himself made frequent use of oaths (Deut., vi. 13; Ps. lxii. 12; II Cor., i. 23; Philem., 8; Apoc., x. 6). The object of the Lord was rather to reprove the perverse opinion of the Jews, which was to the effect that the only thing to be avoided in an oath was a lie. . . . For oaths have been instituted on account of human frailty. They bespeak the inconstancy of him who takes it or the stubbornness of him who refuses to believe without it. However, an oath can be justified by necessity. When Our Lord says, ‘Let your speech be: Yea, yea; No, no,’ He evidently forbids the habit of swearing in familiar conversation and on trivial matters” (“Roman Catechism,” “loc. cit.,” 19).

13. Matt., v. 37.

14. Ecclus., xxiii. 9, 10.

15. Ps. iv. 3.

16. Mark, vi.

17. Jerem., iv. 2. Although to constitute an oath it is sufficient to call God to witness, yet to make a holy and just oath many other conditions are required. . . . The words [of Jeremias, cited above] briefly sum up all the conditions that constitute the perfection of an oath, namely, truth, judgment, justice (“Roman Catechism., “loc. cit.,” 11).

18. Wis., xiii. 1.

19. Levit., xxiv. 16.

20. Deut., vi. 13.

21. Exod., xxiii. 13.

22. Cor., i. 23.

23. I Cor., vi. 11.

24. Jerem., xiv. 9.

25. Isa., iv. 1.

26. Rom., x. 14.

27. “Ibid.,” 13.

28. Isa., xliii. 7.

29. Matt., v. 16.

30. Rom., ii. 24.

31. Prov., xviii. 10.

32. Mark, xvi. 17.

33. Acts, iv. 12.

34. Col., iii. 17.

35. Ps. cxxiii. 8.

36. Eccles., v. 3.

37. Ps. lxxv. 12.

38. Eccles., v. 3.

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