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St. Thomas Aquinas
Catechetical Instructions

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  • SECOND COMMANDMENT: "Thou Shalt Not Take the Name of the Lord Thy God in
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"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.


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.


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