16. Christ as our Redeemer purchased the Church at the price of his own blood; as priest he offered himself, and continues to offer himself as a victim for our sins. Is it not evident, then, that his kingly dignity partakes in a manner of both these offices?
17. It would be a grave error, on the other hand, to say that Christ has no authority whatever in civil affairs, since, by virtue of the absolute empire over all creatures committed to him by the Father, all things are in his power. Nevertheless, during his life on earth he refrained from the exercise of such authority, and although he himself disdained to possess or to care for earthly goods, he did not, nor does he today, interfere with those who possess them. Non eripit mortalia qui regna dat caelestia.
18. Thus the empire of our Redeemer embraces all men. To use the words of Our immortal predecessor, Pope Leo XIII: "His empire includes not only Catholic nations, not only baptized persons who, though of right belonging to the Church, have been led astray by error, or have been cut off from her by schism, but also all those who are outside the Christian faith; so that truly the whole of mankind is subject to the power of Jesus Christ." Nor is there any difference in this matter between the individual and the family or the State; for all men, whether collectively or individually, are under the dominion of Christ. In him is the salvation of the individual, in him is the salvation of society. "Neither is there salvation in any other, for there is no other name under heaven given to men whereby we must be saved." He is the author of happiness and true prosperity for every man and for every nation. "For a nation is happy when its citizens are happy. What else is a nation but a number of men living in concord?" If, therefore, the rulers of nations wish to preserve their authority, to promote and increase the prosperity of their countries, they will not neglect the public duty of reverence and obedience to the rule of Christ. What We said at the beginning of Our Pontificate concerning the decline of public authority, and the lack of respect for the same, is equally true at the present day. "With God and Jesus Christ," we said, "excluded from political life, with authority derived not from God but from man, the very basis of that authority has been taken away, because the chief reason of the distinction between ruler and subject has been eliminated. The result is that human society is tottering to its fall, because it has no longer a secure and solid foundation."
19. 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." 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.
20. If the kingdom of Christ, then, receives, as it should, all nations under its way, there seems no reason why we should despair of seeing that peace which the King of Peace came to bring on earth - he who came to reconcile all things, who came not to be ministered unto but to minister, who, though Lord of all, gave himself to us as a model of humility, and with his principal law united the precept of charity; who said also: "My yoke is sweet and my burden light." Oh, what happiness would be Ours if all men, individuals, families, and nations, would but let themselves be governed by Christ! "Then at length," to use the words addressed by our predecessor, Pope Leo XIII, twenty-five years ago to the bishops of the Universal Church, "then at length will many evils be cured; then will the law regain its former authority; peace with all its blessings be restored. Men will sheathe their swords and lay down their arms when all freely acknowledge and obey the authority of Christ, and every tongue confesses that the Lord Jesus Christ is in the glory of God the Father."
21. That these blessings may be abundant and lasting in Christian society, it is necessary that the kingship of our Savior should be as widely as possible recognized and understood, and to the end nothing would serve better than the institution of a special feast in honor of the Kingship of Christ. For people are instructed in the truths of faith, and brought to appreciate the inner joys of religion far more effectually by the annual celebration of our sacred mysteries than by any official pronouncement of the teaching of the Church. Such pronouncements usually reach only a few and the more learned among the faithful; feasts reach them all; the former speak but once, the latter speak every year - in fact, forever. The church's teaching affects the mind primarily; her feasts affect both mind and heart, and have a salutary effect upon the whole of man's nature. Man is composed of body and soul, and he needs these external festivities so that the sacred rites, in all their beauty and variety, may stimulate him to drink more deeply of the fountain of God's teaching, that he may make it a part of himself, and use it with profit for his spiritual life.
31. When we pay honor to the princely dignity of Christ, men will doubtless be reminded that the Church, founded by Christ as a perfect society, has a natural and inalienable right to perfect freedom and immunity from the power of the state; and that in fulfilling the task committed to her by God of teaching, ruling, and guiding to eternal bliss those who belong to the kingdom of Christ, she cannot be subject to any external power. The State is bound to extend similar freedom to the orders and communities of religious of either sex, who give most valuable help to the Bishops of the Church by laboring for the extension and the establishment of the kingdom of Christ. By their sacred vows they fight against the threefold concupiscence of the world; by making profession of a more perfect life they render the holiness which her divine Founder willed should be a mark and characteristic of his Church more striking and more conspicuous in the eyes of all.
32. Nations will be reminded by the annual celebration of this feast that not only private individuals but also rulers and princes are bound to give public honor and obedience to Christ. It will call to their minds the thought of the last judgment, wherein Christ, who has been cast out of public life, despised, neglected and ignored, will most severely avenge these insults; for his kingly dignity demands that the State should take account of the commandments of God and of Christian principles, both in making laws and in administering justice, and also in providing for the young a sound moral education.
Encyclical On Christ the King by Pope Pius XI 12 Nov 1925