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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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Thus, God forbids adultery both to men and women. Now, it must be known

that, although some believe that adultery is a sin, yet they do not believe

that simple fornication is a mortal sin. Against them stand the words of

St. Paul: "For fornicators and adulterers God will judge."11 And: "Do not

err: neither fornicators, . . . nor adulterers, nor the effeminate, nor

liers with mankind shall possess the kingdom of God."12 But one is not

excluded from the kingdom of God except by mortal sin; therefore,

fornication is a mortal sin.


But one might say that there is no reason why fornication should be a

mortal sin, since the body of the wife is not given, as in adultery. I say,

however, if the body of the wife is not given, nevertheless, there is given

the body of Christ which was given to the husband when he was sanctified in

Baptism. If, then, one must not betray his wife, with much more reason must

he not be unfaithful to Christ: "Know you not that your bodies are the

members of Christ? Shall I then take the members of Christ and make them

the members of a harlot? God forbid!"13 It is heretical to say that

fornication is not a mortal sin.


Moreover, it must be known that the Commandment, "Thou shalt not commit

adultery," not only forbids adultery but also every form of immodesty and

impurity.14 There are some who say that intercourse between married persons

is not devoid of sin. But this is heretical, for the Apostle says: "Let

marriage be honorable in all and the bed undefiled."15 Not only is it

devoid of sin, but for those in the state of grace it is meritorious for

eternal life. Sometimes, however, it may be a venial sin, sometimes a

mortal sin. When it is had with the intention of bringing forth offspring,

it is an act of virtue. When it is had with the intent of rendering mutual

comfort, it is an act of justice. When it is a cause of exciting lust,

although within the limits of marriage, it is a venial sin; and when it

goes beyond these limits, so as to intend intercourse with another if

possible, it would be a mortal sin.


Adultery and fornication are forbidden for a number of reasons. First of

all, because they destroy the soul: "He that is an adulterer, for the folly

of his heart shall destroy his own soul."16 It says: "for the folly of his

heart," which is whenever the flesh dominates the spirit. Secondly, they

deprive one of life; for one guilty of such should die according to the

Law, as we read in Leviticus (xx. 10) and Deuteronomy (xxii. 22). Sometimes

the guilty one is not punished now bodily, which is to his disadvantage

since punishment of the body may be borne with patience and is conducive to

the remission of sins; but nevertheless he shall be punished in the future

life. Thirdly, these sins consume his substance, just as happened to the

prodigal son in that "he wasted his substance living riotiously."17 "Give

not thy soul to harlots in any point; lest thou destroy thyself and thy

inheritance."18 Fourthly, they defile the offspring: "The children of

adulterers shall not come to perfection, and the seed of the unlawful bed

shall be rooted out. And if they live long they shall be nothing regarded,

and their last old age shall be without honor."19 And again: "Otherwise

your children should be unclean; but now they are holy."20 Thus, they are

never honored in the Church, but if they be clerics their dishonor may go

without shame. Fifthly, these sins take away one's honor, and this

especially is applicable to women: "Every woman that is a harlot shall be

trodden upon as dung in the way."21 And of the husband it is said: "He

gathereth to himself shame and dishonor, and his reproach shall not be

blotted out."22


St. Gregory says that sins of the flesh are more shameful and less blameful

than those of the spirit, and the reason is because they are common to the

beasts: "Man when he was in honor did not understand; and he hath been

compared to senseless beasts, and made like to them."23






11. Heb., xiii. 4.


12. I Cor., vi. 9.


13. I Cor., vi. 15.


14. "By the prohibition of adultery, every kind of impurity and immodesty

by which the body is defiled is also forbidden. Nay more, even every inward

thought against chastity is forbidden by this Commandment. . . . You have

heard that it was said to them of old: Thou shalt not commit adultery. But

I say to you, that whcsoever shall look on a woman to lust after her, hath

already committed adultery with her in his heart." ("Roman Catechism,"

"loc. cit.," 5).


15. Heb., xiii. 4.


16. Prov., vi. 32.


17. Luke, xv. 13.


18. Ecclus., ix. 6.


19. Wis., iii. 16-17.


20. I Cor., vii. 14.


21. Ecclus., ix. 10.


22. Prov., vi. 33.


23. Ps.xlviii. 21. "If the occasions of sin which we have just enumerated

[viz., idleness, intemperance in eating and drinking, indulgence of the

eyes, immodest dress, immodest conversation and reading] be carefully

avoided, almost every excitement to lust will be removed. But the most

efficacious means to subdue its violence are frequent use of confession and

reception of the Holy Eucharist. Unceasing and devout prayer to God,

accompanied by fasting and giving of alms, has the same salutary effect.

Chastity is a gilt of God. To those who ask it aright, He does not deny it;

nor does He allow us to be tempted beyond our strength" ("Roman Catechism,"

"loc. cit.," 12).

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