From Pope Pius XI’s encyclical Quas Primas (1925):
When once men recognize, both in private and in public life, that Christ is King, society will at last receive the great blessings of real liberty, well-ordered discipline, peace and harmony. Our Lord’s regal office invests the human authority of princes and rulers with a religious significance; it ennobles the citizen’s duty of obedience. It is for this reason that St. Paul, while bidding wives revere Christ in their husbands, and slaves respect Christ in their masters, warns them to give obedience to them not as men, but as the vicegerents of Christ; for it is not meet that men redeemed by Christ should serve their fellow-men. “You are bought with a price; be not made the bond-slaves of men.”32 If princes and magistrates duly elected are filled with the persuasion that they rule, not by their own right, but by the mandate and in the place of the Divine King, they will exercise their authority piously and wisely, and they will make laws and administer them, having in view the common good and also the human dignity of their subjects. The result will be a stable peace and tranquillity, for there will be no longer any cause of discontent. Men will see in their king or in their rulers men like themselves, perhaps unworthy or open to criticism, but they will not on that account refuse obedience if they see reflected in them the authority of Christ God and Man. Peace and harmony, too, will result; for with the spread and the universal extent of the kingdom of Christ men will become more and more conscious of the link that binds them together, and thus many conflicts will be either prevented entirely or at least their bitterness will be diminished.
32 I Cor. vii, 23.
— Quas Primas 19