Here are some highlights from the encyclical Quas Primas of Pope Pius XI. He published it in 1925, and in it, he instituted the feast of Christ the King.
Christ the King painting in the Sainte Chapelle, Paris. Image by Lawrence Lew OP, CC-BY-NC-ND 2.0, here
7. It has long been a common custom to give to Christ the metaphorical title of “King,” because of the high degree of perfection whereby he excels all creatures. So he is said to reign “in the hearts of men,” both by reason of the keenness of his intellect and the extent of his knowledge, and also because he is very truth, and it is from him that truth must be obediently received by all mankind. He reigns, too, in the wills of men, for in him the human will was perfectly and entirely obedient to the Holy Will of God, and further by his grace and inspiration he so subjects our free-will as to incite us to the most noble endeavors. He is King of hearts, too, by reason of his “charity which exceedeth all knowledge.”
13. The foundation of this power and dignity of Our Lord is rightly indicated by Cyril of Alexandria. “Christ,” he says, “has dominion over all creatures, a dominion not seized by violence nor usurped, but his by essence and by nature.” His kingship is founded upon the ineffable hypostatic union. From this it follows not only that Christ is to be adored by angels and men, but that to him as man angels and men are subject, and must recognize his empire; by reason of the hypostatic union Christ has power over all creatures. But a thought that must give us even greater joy and consolation is this that Christ is our King by acquired, as well as by natural right, for he is our Redeemer. Would that they who forget what they have cost their Savior might recall the words: “You were not redeemed with corruptible things, but with the precious blood of Christ, as of a lamb unspotted and undefiled.” We are no longer our own property, for Christ has purchased us “with a great price”; our very bodies are the “members of Christ.”
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. […] 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. […] 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. […]
24. […] But if the faithful were generally to understand that it behooves them ever to fight courageously under the banner of Christ their King, then, fired with apostolic zeal, they would strive to win over to their Lord those hearts that are bitter and estranged from him, and would valiantly defend his rights.
25. Moreover, the annual and universal celebration of the feast of the Kingship of Christ will draw attention to the evils which anticlericalism has brought upon society in drawing men away from Christ, and will also do much to remedy them. While nations insult the beloved name of our Redeemer by suppressing all mention of it in their conferences and parliaments, we must all the more loudly proclaim his kingly dignity and power, all the more universally affirm his rights.
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. […]
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.