I answer that, Unbelief may be taken in two ways: first, by way of pure negation, so that a man be called an unbeliever, merely because he has not the faith. Secondly, unbelief may be taken by way of opposition to the faith; in which sense a man refuses to hear the faith, or despises it, according to Is. 53:1: "Who hath believed our report?" It is this that completes the notion of unbelief, and it is in this sense that unbelief is a sin.
If, however, we take it by way of pure negation, as we find it in those who have heard nothing about the faith, it bears the character, not of sin, but of punishment, because such like ignorance of Divine things is a result of the sin of our first parent. If such like unbelievers are damned, it is on account of other sins, which cannot be taken away without faith, but not on account of their sin of unbelief. Hence Our Lord said (Jn. 15:22) "If I had not come, and spoken to them, they would not have sin"; which Augustine expounds (Tract. lxxxix in Joan.) as "referring to the sin whereby they believed not in Christ."
2. Whether unbelief is the greatest of sins?
I answer that, Every sin consists formally in aversion from God, as stated above (FS, Q[71], A[6]; FS, Q[73], A[3]). Hence the more a sin severs man from God, the graver it is. Now man is more than ever separated from God by unbelief, because he has not even true knowledge of God: and by false knowledge of God, man does not approach Him, but is severed from Him.
Nor is it possible for one who has a false opinion of God, to know Him in any way at all, because the object of his opinion is not God. Therefore it is clear that the sin of unbelief is greater than any sin that occurs in the perversion of morals. This does not apply to the sins that are opposed to the theological virtues, as we shall stated further on (Q[20], A[3]; Q[34], A[2], ad 2; Q[39], A[2], ad 3).
3. Whether unbelievers should have authority or dominion over the faithful?
I answer that, That this question may be considered in two ways. First, we may speak of dominion or authority of unbelievers over the faithful as of a thing to be established for the first time. This ought by no means to be allowed, since it would provoke scandal and endanger the faith, for subjects are easily influenced by their superiors to comply with their commands, unless the subjects are of great virtue: moreover unbelievers hold the faith in contempt, if they see the faithful fall away. Hence the Apostle forbade the faithful to go to law before an unbelieving judge. And so the Church altogether forbids unbelievers to acquire dominion over believers, or to have authority over them in any capacity whatever.
Secondly, we may speak of dominion or authority, as already in force: and here we must observe that dominion and authority are institutions of human law, while the distinction between faithful and unbelievers arises from the Divine law. Now the Divine law which is the law of grace, does not do away with human law which is the law of natural reason. Wherefore the distinction between faithful and unbelievers, considered in itself, does not do away with dominion and authority of unbelievers over the faithful.
Nevertheless this right of dominion or authority can be justly done away with by the sentence or ordination of the Church who has the authority of God: since unbelievers in virtue of their unbelief deserve to forfeit their power over the faithful who are converted into children of God.
This the Church does sometimes, and sometimes not. For among those unbelievers who are subject, even in temporal matters, to the Church and her members, the Church made the law that if the slave of a Jew became a Christian, he should forthwith receive his freedom, without paying any price, if he should be a "vernaculus," i.e. born in slavery; and likewise if, when yet an unbeliever, he had been bought for his service: if, however, he had been bought for sale, then he should be offered for sale within three months. Nor does the Church harm them in this, because since those Jews themselves are subject to the Church, she can dispose of their possessions, even as secular princes have enacted many laws to be observed by their subjects, in favor of liberty. On the other hand, the Church has not applied the above law to those unbelievers who are not subject to her or her members, in temporal matters, although she has the right to do so: and this, in order to avoid scandal, for as Our Lord showed (Mat. 17:25,26) that He could be excused from paying the tribute, because "the children are free," yet He ordered the tribute to be paid in order to avoid giving scandal. Thus Paul too, after saying that servants should honor their masters, adds, "lest the name of the Lord and His doctrine be blasphemed."
4. Whether heretics should be tolerated?
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."