From the Encyclical of Pius XI, Quas Primas:
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.