[In the excerpts below from Summa Theologica (written 1265-1274), St. Thomas Aquinas takes up two questions: Whether it is lawful to kill sinners? and Whether heretics ought to be tolerated? A PDF version of this text is here.]
Whether it is lawful to kill sinners?
Objection 1. It would seem unlawful to kill men who have sinned. For our Lord in the parable (Mt. 13) forbade the uprooting of the cockle which denotes wicked men according to a gloss. Now whatever is forbidden by God is a sin. Therefore it is a sin to kill a sinner.
Objection 2. Further, human justice is conformed to Divine justice. Now according to Divine justice sinners are kept back for repentance, according to Ezech. 33:11, “I desire not the death of the wicked, but that the wicked turn from his way and live.” Therefore it seems altogether unjust to kill sinners.
Objection 3. Further, it is not lawful, for any good end whatever, to do that which is evil in itself, according to Augustine (Contra Mendac. vii) and the Philosopher (Ethic. ii, 6). Now to kill a man is evil in itself, since we are bound to have charity towards all men, and “we wish our friends to live and to exist,” according to Ethic. ix, 4. Therefore it is nowise lawful to kill a man who has sinned.
On the contrary, It is written (Ex. 22:18): “Wizards thou shalt not suffer to live”; and (Ps. 100:8): “In the morning I put to death all the wicked of the land.”
I answer that, As stated above (1), it is lawful to kill dumb animals, in so far as they are naturally directed to man’s use, as the imperfect is directed to the perfect. Now every part is directed to the whole, as imperfect to perfect, wherefore every part is naturally for the sake of the whole. For this reason we observe that if the health of the whole body demands the excision of a member, through its being decayed or infectious to the other members, it will be both praiseworthy and advantageous to have it cut away. Now every individual person is compared to the whole community, as part to whole. Therefore if a man be dangerous and infectious to the community, on account of some sin, it is praiseworthy and advantageous that he be killed in order to safeguard the common good, since “a little leaven corrupteth the whole lump” (1 Cor. 5:6).
Reply to Objection 1. Our Lord commanded them to forbear from uprooting the cockle in order to spare the wheat, i.e. the good. This occurs when the wicked cannot be slain without the good being killed with them, either because the wicked lie hidden among the good, or because they have many followers, so that they cannot be killed without danger to the good, as Augustine says (Contra Parmen. iii, 2). Wherefore our Lord teaches that we should rather allow the wicked to live, and that vengeance is to be delayed until the last judgment, rather than that the good be put to death together with the wicked. When, however, the good incur no danger, but rather are protected and saved by the slaying of the wicked, then the latter may be lawfully put to death.
Reply to Objection 2. According to the order of His wisdom, God sometimes slays sinners forthwith in order to deliver the good, whereas sometimes He allows them time to repent, according as He knows what is expedient for His elect. This also does human justice imitate according to its powers; for it puts to death those who are dangerous to others, while it allows time for repentance to those who sin without grievously harming others.
Reply to Objection 3. By sinning man departs from the order of reason, and consequently falls away from the dignity of his manhood, in so far as he is naturally free, and exists for himself, and he falls into the slavish state of the beasts, by being disposed of according as he is useful to others. This is expressed in Ps. 48:21: “Man, when he was in honor, did not understand; he hath been compared to senseless beasts, and made like to them,” and Prov. 11:29: “The fool shall serve the wise.” Hence, although it be evil in itself to kill a man so long as he preserve his dignity, yet it may be good to kill a man who has sinned, even as it is to kill a beast. For a bad man is worse than a beast, and is more harmful, as the Philosopher states (Polit. i, 1 and Ethic. vii, 6).
Whether heretics ought to be tolerated?
Objection 1. It seems that heretics ought to be tolerated. For the Apostle says (2 Tim. 2:24,25): “The servant of the Lord must not wrangle … with modesty admonishing them that resist the truth, if peradventure God may give them repentance to know the truth, and they may recover themselves from the snares of the devil.” Now if heretics are not tolerated but put to death, they lose the opportunity of repentance. Therefore it seems contrary to the Apostle’s command.
Objection 2. Further, whatever is necessary in the Church should be tolerated. Now heresies are necessary in the Church, since the Apostle says (1 Cor. 11:19): “There must be … heresies, that they … who are reproved, may be manifest among you.” Therefore it seems that heretics should be tolerated.
Objection 3. Further, the Master commanded his servants (Mt. 13:30) to suffer the cockle “to grow until the harvest,” i.e. the end of the world, as a gloss explains it. Now holy men explain that the cockle denotes heretics. Therefore heretics should be tolerated.
On the contrary, The Apostle says (Titus 3:10,11): “A man that is a heretic, after the first and second admonition, avoid: knowing that he, that is such an one, is subverted.”
I answer that, With regard to heretics two points must be observed: one, on their own side; the other, on the side of the Church. On their own side there is the sin, whereby they deserve not only to be separated from the Church by excommunication, but also to be severed from the world by death. For it is a much graver matter to corrupt the faith which quickens the soul, than to forge money, which supports temporal life. Wherefore if forgers of money and other evil-doers are forthwith condemned to death by the secular authority, much more reason is there for heretics, as soon as they are convicted of heresy, to be not only excommunicated but even put to death.
On the part of the Church, however, there is mercy which looks to the conversion of the wanderer, wherefore she condemns not at once, but “after the first and second admonition,” as the Apostle directs: after that, if he is yet stubborn, the Church no longer hoping for his conversion, looks to the salvation of others, by excommunicating him and separating him from the Church, and furthermore delivers him to the secular tribunal to be exterminated thereby from the world by death. For Jerome commenting on Gal. 5:9, “A little leaven,” says: “Cut off the decayed flesh, expel the mangy sheep from the fold, lest the whole house, the whole paste, the whole body, the whole flock, burn, perish, rot, die. Arius was but one spark in Alexandria, but as that spark was not at once put out, the whole earth was laid waste by its flame.”
Reply to Objection 1. This very modesty demands that the heretic should be admonished a first and second time: and if he be unwilling to retract, he must be reckoned as already subverted,” as we may gather from the words of the Apostle quoted above.
Reply to Objection 2. The profit that ensues from heresy is beside the intention of heretics, for it consists in the constancy of the faithful being put to the test, and “makes us shake off our sluggishness, and search the Scriptures more carefully,” as Augustine states (De Gen. cont. Manich. i, 1). What they really intend is the corruption of the faith, which is to inflict very great harm indeed. Consequently we should consider what they directly intend, and expel them, rather than what is beside their intention, and so, tolerate them.
Reply to Objection 3. According to Decret. (xxiv, qu. iii, can. Notandum), “to be excommunicated is not to be uprooted.” A man is excommunicated, as the Apostle says (1 Cor. 5:5) that his “spirit may be saved in the day of Our Lord.” Yet if heretics be altogether uprooted by death, this is not contrary to Our Lord’s command, which is to be understood as referring to the case when the cockle cannot be plucked up without plucking up the wheat, as we explained above (10, 8, ad 1), when treating of unbelievers in general.
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[A PDF version of this text is here.]