Objection 1. It would seem that unbelievers may have authority or dominion over the faithful. For the Apostle says (1 Timothy 6:1): "Whosoever are servants under the yoke, let them count their masters worthy of all honor": and it is clear that he is speaking of unbelievers, since he adds (1 Timothy 6:2): "But they that have believing masters, let them not despise them." Moreover it is written (1 Peter 2:18): "Servants be subject to your masters with all fear, not only to the good and gentle, but also to the froward." Now this command would not be contained in the apostolic teaching unless unbelievers could have authority over the faithful. Therefore it seems that unbelievers can have authority over the faithful.
Objection 2. Further, all the members of a prince's household are his subjects. Now some of the faithful were members of unbelieving princes' households, for we read in the Epistle to the Philippians (4:22): "All the saints salute you, especially they that are of Caesar's household," referring to Nero, who was an unbeliever. Therefore unbelievers can have authority over the faithful.
Objection 3. Further, according to the Philosopher (Polit. i, 2) a slave is his master's instrument in matters concerning everyday life, even as a craftsman's laborer is his instrument in matters concerning the working of his art. Now, in such matters, a believer can be subject to an unbeliever, for he may work on an unbeliever's farm. Therefore unbelievers may have authority over the faithful even as to dominion.
On the contrary, Those who are in authority can pronounce judgment on those over whom they are placed. But unbelievers cannot pronounce judgment on the faithful, for the Apostle says (1 Corinthians 6:1): "Dare any of you, having a matter against another, go to be judged before the unjust," i.e. unbelievers, "and not before the saints?" Therefore it seems that unbelievers cannot have authority 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 (Matthew 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."
This suffices for the Reply to the First Objection.
Reply to Objection 2. The authority of Caesar preceded the distinction of faithful from unbelievers. Hence it was not cancelled by the conversion of some to the faith. Moreover it was a good thing that there should be a few of the faithful in the emperor's household, that they might defend the rest of the faithful. Thus the Blessed Sebastian encouraged those whom he saw faltering under torture, and, the while, remained hidden under the military cloak in the palace of Diocletian.
Reply to Objection 3. Slaves are subject to their masters for their whole lifetime, and are subject to their overseers in everything: whereas the craftsman's laborer is subject to him for certain special works. Hence it would be more dangerous for unbelievers to have dominion or authority over the faithful, than that they should be allowed to employ them in some craft. Wherefore the Church permits Christians to work on the land of Jews, because this does not entail their living together with them. Thus Solomon besought the King of Tyre to send master workmen to hew the trees, as related in 1 Kings 5:6. Yet, if there be reason to fear that the faithful will be perverted by such communications and dealings, they should be absolutely forbidden.
Source: Summa Theologica (New Advent)