Objection 1: It seems that heretics must be tolerated. For the Apostle says (2 Tim 2:24-25): “The servant of the Lord must not strive… but in meekness instruct those that oppose themselves; if God peradventure will give them repentance to the acknowledging of the truth, and that they may recover themselves out of the snare of the devil.” Now, if heretics are not tolerated but put to death, they lose the opportunity for repentance. Therefore, this seems to be contrary to the command of the Apostle.
Objection 2: Furthermore, 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 also be heresies among you, that they which are approved may be made manifest among you.” Therefore, it seems that heretics should be tolerated.
Objection 3: Furthermore, the Lord commanded His servants (Mt 13:30) to “let both grow together until the harvest,” that is, until the end of the world, as a gloss explains. Now holy men interpret the tares as referring to 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, reject; knowing that he that is such is subverted.”
I answer that: In regard to heretics, two points must be considered: one on their own side, the other on the side of the Church. On their own side is the sin, by which 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 gives life to the soul, than to counterfeit money, which supports temporal life. Therefore, if forgers of money and other malefactors are at once condemned to death by the secular authority, there is even greater reason for heretics, as soon as they are convicted of heresy, not only to be excommunicated but also to be put to death. On the side of the Church, however, there is mercy, which looks to the conversion of the erring; therefore, she does not at once condemn, but “after the first and second admonition,” as the Apostle directs; afterwards, if he is still 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 further, she hands him over to the secular tribunal to be exterminated from the world by death. For Jerome says in his commentary on Gal. 5:9, “A little leaven”: Cut off the decayed flesh, drive out the scabby sheep from the fold, lest the whole house, the whole dough, the whole body, the whole flock burn, perish, rot, die. Arius was only a spark in Alexandria, but because that spark was not at once extinguished, the whole earth was devastated by his flame.
Reply to Objection 1: This very meekness requires that the heretic be admonished once and a second time; and if he is unwilling to recant, he must be considered as already “perverted,” as we gather from the Apostle’s words quoted above.
Reply to Objection 2: The benefit that follows from heresy lies outside the intention of the heretics, for it consists in the steadfastness of the faithful, who are put to the test, and “makes us shake off our laziness and search the Scriptures more carefully,” as Augustine notes (De Gen. cont. Manich. i, 1). What they really intend is the corruption of the faith, which means causing very great harm. Consequently, we should consider what they directly intend and expel them, rather than tolerate them for the sake of what lies outside their intention.
Reply to Objection 3: According to Decret. (xxiv, qu. ii, can. Notandum), “to be excommunicated is not to be uprooted.” A man is excommunicated, as the Apostle says (1 Cor 5:5), so that his “spirit may be saved in the day of our Lord.” But if heretics are utterly uprooted by death, this is not against the command of our Lord, which is to be understood as referring to the case when the tares cannot be plucked up without uprooting the wheat, as we explained above (Question [10], Article [8], ad 1), when we were treating of unbelievers in general.
Source: Summa Theologiae, Second Part of the Second Part, Question 11, Article 3