Article 4 — Whether it is a mortal sin to have intercourse with a concubine?

Objection 1: It would seem that it is not a mortal sin to have intercourse with a concubine. For a lie is a greater sin than simple fornication: and a proof of this is that Judah, who did not abhor to commit fornication with Tamar, recoiled from telling a lie, saying: surely she cannot charge us with a lie (Gen 38:23). But a lie is not always a mortal sin. Neither, therefore, is simple fornication.

Obj. 2: Further, a mortal sin should be punished with death. But the old law did not punish with death intercourse with a concubine, save in a certain case (Deut 22:25). Therefore, it is not a mortal sin.

Obj. 3: Further, according to Gregory (Morals on Job 33.12), the sins of the flesh are less blameworthy than spiritual sins. Now pride and covetousness, which are spiritual sins, are not always mortal sins. Therefore, fornication, which is a sin of the flesh, is not always a mortal sin.

Obj. 4: Further, where the incentive is greater, the sin is less grievous, because he sins more who is overcome by a lighter temptation. But concupiscence is the greatest incentive to lust. Therefore, since lustful actions are not always mortal sins, neither is simple fornication a mortal sin.

On the contrary, Nothing but mortal sin excludes from the kingdom of God. But fornicators are excluded from the kingdom of God (1 Cor 6:9–10). Therefore, simple fornication is a mortal sin.

Further, Mortal sins alone are called crimes. Now all fornication is a crime, according to Tobit 4:13: take heed to keep yourself from all fornication, and beside your wife never endure to know crime. Therefore, etc.

I answer that, As we have already stated (Sentences II, D. 42, Q. 1, A. 4), those sins are mortal in their genus which violate the bond of friendship between man and God, and between man and man; for such sins are against the two precepts of charity, which is the life of the soul. Therefore, since the intercourse of fornication destroys the due relations of the parent with the offspring that is nature’s aim in sexual intercourse, there can be no doubt that simple fornication by its very nature is a mortal sin, even though there were no written law.

Reply Obj. 1: It often happens that a man who does not avoid a mortal sin avoids a venial sin to which he has not so great an incentive. Thus, too, Judah avoided a lie while he avoided not fornication. Nevertheless, that would have been a pernicious lie, for it would have involved an injury if he had not kept his promise.

Reply Obj. 2: A sin is called mortal not because it is punished with temporal, but because it is punished with eternal death. Hence also theft, which is a mortal sin, and many other sins are sometimes not punished with temporal death by the law. The same applies to fornication.

Reply Obj. 3: Just as not every movement of pride is a mortal sin, so neither is every movement of lust, because the first movements of lust and the like are venial sins, even sometimes marriage intercourse. Nevertheless, some acts of lust are mortal sins, while some movements of pride are venial: since the words quoted from Gregory are to be understood as comparing vices in their genus and not in their particular acts.

Reply Obj. 4: A circumstance is the more effective in aggravating a sin according as it comes nearer to the nature of sin. Thus, although fornication is less grave on account of the greatness of its incentive, yet on account of the matter about which it is, it has a greater gravity than immoderate eating, because it is about those things which tighten the bond of human fellowship, as stated above. Hence the argument does not prove.