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St. Thomas Aquinas
Catechetical Instructions

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  • THE SIXTH COMMANDMENT: "Thou Shalt Not Commit Adultery."
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THE SIXTH COMMANDMENT: "Thou Shalt Not Commit Adultery."


After the prohibition of murder, adultery is forbidden. This is fitting,

since husband and wife are as one body. "They shall be," says the Lord,

"two in one flesh."1 Therefore, after an injury inflicted upon a man in his

own person, none is so grave as that which is inflicted upon a person with

whom one is joined.2


Adultery is forbidden both to the wife and the husband. We shall first

consider the adultery of the wife, since in this seems to lie the greater

sin, for a wife who commits adultery is guilty of three grave sins, which

are implied in the following words: "So every woman that leaveth her

husband, . . . first, she hath been unfaithful to the law of the Most High;

and secondly, she hath offended against her husband; thirdly, she hath

fornicated in adultery, and hath gotten her children of another man."


First, therefore, she has sinned by lack of faith, since she is unfaithful

to the law wherein God has forbidden adultery. Moreover, she has spurned

the ordinance of God: "What therefore God has joined together, let no man

put asunder."4 And also she has sinned against the institution or

Sacrament. Because marriage is contracted before the eyes of the Church,

and thereupon God is called, as it were, to witness a bond of fidelity

which must be kept: "The Lord hath been witness between thee and the wife

of thy youth whom thou hast despised."5 Therefore, she has sinned against

the law of God, against a precept of the Church and against a Sacrament of



Secondly, she sins by infidelity because she has betrayed her husband: "The

wife hath not power of her own body: but the husband."6 In fact, without

the consent of the husband she cannot observe chastity. If adultery is

committed, then, an act of treachery is perpetrated in that the wife gives

herself to another, just as if a servant gave himself to another master:

"She forsaketh the guide of her youth, and hath forgotten the covenant of

her God."7


Thirdly, the adulteress commits the sin of theft in that she brings forth

children from a man not her husband; and this is a most grave theft in that

she expends her heredity upon children not her husband's. Let it be noted

that such a one should encourage her children to enter religion, or upon

such a walk of life that they do not succeed in the property of her

husband. Therefore, an adulteress is guilty of sacrilege, treachery and



Husbands, however, do not sin any less than wives, although they sometimes

may salve themselves to the contrary. This is clear for three reasons.

First, because of the equality which holds between husband and wife, for

"the husband also hath not power of his own body, but the wife."8

Therefore, as far as the rights of matrimony are concerned, one cannot act

without the consent of the other. As an indication of this, God did not

form woman from the foot or from the head, but from the rib of the man.

Now, marriage was at no time a perfect state until the law of Christ came,

because the Jew could have many wives, but a wife could not have many

husbands; hence, equality did not exist.


The second reason is because strength is a special quality of the man,

while the passion proper to the woman is concupiscence: "Ye husbands,

likewise dwelling with them according to knowledge, giving honor to the

female as to the weaker vessel."9 Therefore, if you ask from your wife what

you do not keep yourself, then you are unfaithful. The third reason is from

the authority of the husband. For the husband is head of the wife, and as

it is said: "Women may not speak in the church, . . . if they would learn

anything, let them ask their husbands at home."10 The husband is the

teacher of his wife, and God, therefore, gave the Commandment to the

husband. Now, as regards fulfillment of their duties, a priest who fails is

more guilty than a layman, and a bishop more than a priest, because it is

especially incumbent upon them to teach others. In like manner, the husband

that commits adultery breaks faith by not obeying that which he ought.


1. Gen., ii. 24.


2. "The bond between husband and wife is one of the strictest union, and

nothing can be more gratifying to both than to realize that they are

objects of mutual and undivided affection. On the other hand, nothing

inflicts greater anguish than to feel that the legitimate love which they

owe to each other has been transferred elsewhere. This Commandment which

prohibits adultery follows properly and in order that which protects human

life against the hand of the murderer" ("Roman Catechism," "Sixth

Commandment," 1). St. Thomas treats of this Commandment also in the "Summa

Theol.," II-II, Q. cxxii, art. 6; Q. cliv.



4. Matt., xix. 6.


5Mal., ii. 14.


6. Cor., vii. 4.


7. Prov., ii. 17-18.


8. I Cor., vii. 4.


9. I Peter, iii. 7.


10. I Cor., xiv. 34-35.


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