In his Encyclical of December 11, 1925, Pope Pius XI denounced the great modern heresy of secularism. It refuses to recognize the rights of God and His Christ over persons and over society itself, as though God did not exist.
The Holy Father thus instituted the feast of Christ the King to be a public, social and official declaration of the royal rights of Jesus, as God the Creator, as The Word Incarnate, and as Redeemer. This feast makes these rights to be known and recognized, in a way most suitable to man and to society by the sublimest acts of religion, particularly by Holy Mass. In fact, the end of the Holy Sacrifice is the acknowledgment of God's complete dominion over us, and our complete dependence on Him.
The Holy Father expressed his wish that this feast should be celebrated towards the end of the liturgical year, on the last Sunday of October, as the consummation of all the mysteries by which Jesus has established His royal powers and nearly on the eve of All Saints, where He already realizes them in part in being "the crown of all saints"; until He shall be the crown of all those on earth whom He saves by the application of the merits of His Passion in the Mass (Secret).
The end of the Eucharist, says the Catechism of the Council of Trent, is "to form one sole mystic body of all the faithful" and so to draw them in the worship which Christ, king-adorer, as priest and victim, rendered in a bloody manner on the cross and now renders, in an unbloody manner, on the stone altar of our churches and on the golden altar in heaven, to Christ, king-adored, as Son of God, and to His Father to whom He offers these souls (Preface).
Source: Dom Gaspar Lefebvre, OSB, 1945, adapted and abridged.