8 As with individuals, so with nations. These, too, must necessarily tend to ruin if they go astray from "The Way." The Son of God, the Creator and Redeemer of mankind, is King and Lord of the earth, and holds supreme dominion over men, both individually and collectively. "And He gave Him power, and glory, and a kingdom: and all peoples, tribes, and tongues shall serve Him" (Daniel VII., 14). "I am appointed King by Him . . . I will give Thee the Gentiles for Thy inheritance, and the uttermost parts of the earth for Thy possession" (Psalm II., 6, 8). Therefore the law of Christ ought to prevail in human society and be the guide and teacher of public as well as of private life. Since this is so by divine decree, and no man may with impunity contravene it, it is an evil thing for the common weal wherever Christianity does not hold the place that belongs to it. When Jesus Christ is absent, human reason fails, being bereft of its chief protection and light, and the very end is lost sight of, for which, under God's providence, human society has been built up. This end is the obtaining by the members of society of natural good through the aid of civil unity, though always in harmony with the perfect and eternal good which is above nature. But when men's minds are clouded, both rulers and ruled go astray, for they have no safe line to follow nor end to aim at.
Source: Tametsi Futura Prospicientibus (Vatican.va)