I would like to quote paragraph 18 in its entirety.
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."[ Enc. Annum Sacrum, May 25, 1899] 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." [Acts 4:12] 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?"[ S. Aug. Ep. ad Macedonium, c. iii] 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."[ Enc. Ubi Arcano]
In our country, how far have we wandered from this very salient point regarding the necessity of the rulers of nations to not neglect the public duty of reverence and obedience to the rule of Christ? Do we see our society tottering to its fall because of a lack of this secure and solid foundation? The beginning of paragraph 19 sums up what we can expect if we honor Christ as King.
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.
Unfortunately, it often appears that our society is headed in the opposite direction of experiencing these blessings. We continue to remove God from more and more of our public institutions and we are subsequently reaping a lack of discipline, peace and harmony. The polarization of viewpoints on issues such as abortion, same-sex marriage, and religious liberty shows the divide growing deeper and wider with each passing year. Peace and harmony doesn’t seem to be the main item on the menu in our times.
It is important for people of faith to continue to honor Jesus Christ as King, both in their private life as well as their public life. The Church needs faithful witnesses. Obviously, Pope Pius XI recognized some troubling societal signs in his time; many of these signs are still with us today. We can’t give up hope. During this Advent season, let us be more diligent than ever in preparing the way of the Lord. May Christ the King reign supreme in our hearts and homes this day and always.