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 (Mt. 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.