Objection 1. It seems that Christians are not bound to obey the secular power. For a gloss on Matthew 17:25, "Then the children are free," says: "If in every kingdom the children of the king who holds sway over that kingdom are free, then the children of that King, under Whose sway are all kingdoms, should be free in every kingdom." Now Christians, by their faith in Christ, are made children of God, according to John 1:12: "He gave them power to be made the sons of God, to them that believe in His name." Therefore they are not bound to obey the secular power.
Objection 2. Further, it is written (Romans 7:4): "You... are become dead to the law by the body of Christ," and the law mentioned here is the divine law of the Old Testament. Now human law whereby men are subject to the secular power is of less account than the divine law of the Old Testament. Much more, therefore, since they have become members of Christ's body, are men freed from the law of subjection, whereby they were under the power of secular princes.
Objection 3. Further, men are not bound to obey robbers, who oppress them with violence. Now, Augustine says (De Civ. Dei iv): "Without justice, what else is a kingdom but a huge robbery?" Since therefore the authority of secular princes is frequently exercised with injustice, or owes its origin to some unjust usurpation, it seems that Christians ought not to obey secular princes.
On the contrary, It is written (Titus 3:1): "Admonish them to be subject to princes and powers," and (1 Peter 2:13-14): "Be ye subject... to every human creature for God's sake: whether it be to the king as excelling, or to governors as sent by him."
I answer that, Faith in Christ is the origin and cause of justice, according to Romans 3:22, "The justice of God by faith of Jesus Christ:" wherefore faith in Christ does not void the order of justice, but strengthens it." Now the order of justice requires that subjects obey their superiors, else the stability of human affairs would cease. Hence faith in Christ does not excuse the faithful from the obligation of obeying secular princes.
Reply to Objection 1. As stated above (Article 5), subjection whereby one man is bound to another regards the body; not the soul, which retains its liberty. Now, in this state of life we are freed by the grace of Christ from defects of the soul, but not from defects of the body, as the Apostle declares by saying of himself (Romans 7:23) that in his mind he served the law of God, but in his flesh the law of sin. Wherefore those that are made children of God by grace are free from the spiritual bondage of sin, but not from the bodily bondage, whereby they are held bound to earthly masters, as a gloss observes on 1 Timothy 6:1, "Whosoever are servants under the yoke," etc.
Reply to Objection 2. The Old Law was a figure of the New Testament, and therefore it had to cease on the advent of truth. And the comparison with human law does not stand because thereby one man is subject to another. Yet man is bound by divine law to obey his fellow-man.
Reply to Objection 3. Man is bound to obey secular princes in so far as this is required by order of justice. Wherefore if the prince's authority is not just but usurped, or if he commands what is unjust, his subjects are not bound to obey him, except perhaps accidentally, in order to avoid scandal or danger.
Source: Summa Theologica (New Advent)