No doubt the apostle admonishes the Romans to be subject to all power, because there is no power but of God, and because (the ruler) does not carry the sword without reason, and is the servant of God, nay also, says he, a revenger to execute wrath upon him that does evil. For he had also previously spoken thus: “For rulers are not a terror to a good work, but to an evil. Will you then not be afraid of the power? Do that which is good, and you shall have praise of it. Therefore he is a minister of God to you for good.
But if you do that which is evil, be afraid.” Thus he bids you be subject to the powers, not on an opportunity occurring for his avoiding martyrdom, but when he is making an appeal in behalf of a good life, under the view also of their being as it were assistants bestowed upon righteousness, as it were handmaids of the divine court of justice, which even here pronounces sentence beforehand upon the guilty. Then he goes on also to show how he wishes you to be subject to the powers, bidding you pay “tribute to whom tribute is due, custom to whom custom,” that is, the things which are Cæsar's to Cæsar, and the things which are God's to God; but man is the property of God alone.
Peter, no doubt, had likewise said that the king indeed must be honoured, yet so that the king be honoured only when he keeps to his own sphere, when he is far from assuming divine honours; because both father and mother will be loved along with God, not put on an equality with Him. Besides, one will not be permitted to love even life more than God.
Source: Scorpiace (New Advent)