From the encyclical Quas Primas by Pope Pius XI, instituting the Feast of Christ the King:
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.