St. Augustine taught that mankind is divided into two groups or “cities,” the City of God and the City of Man. Inhabitants of the City of God have their will and reason turned toward communion with God. The City of Man looks to purely earthly ends, yet these groups are “commingled” in this life, just as the wheat and the chaff grow in the same field until the Judgment Day.
This creates a pluralism of moral and religious values that will persist until the end of the age. There can be no common agreement as to every right and wrong, except for a common desire for “the sweetness of peace which is dear to all,” and the “tranquility of order.” [City of God, XIX:11, 13]
Here again, one sees that same common ground inherent in the non-aggression principle. Augustine wrote that, “peace is a good so great, that even in this earthly and mortal life there is no word we hear with such pleasure, nothing we more strongly desire, or enjoy more thoroughly when it comes.” [City of God, XIX:11.] The citizens of both cities have a common interest in peace. They can agree, at a minimum, upon a secure and orderly society that lets them pursue their goals, be they spiritual or material.
When laws truly protect us from harming one another, we respect those laws so unanimously that even criminals will agree (at least as those protections apply to themselves). On the other hand, when we cannot agree on moral values that do not impinge directly on others, we lose respect for the law. It seems brutal, rather than just and leads to contempt for the law.
Libertarians, along with St. Augustine, see the wisdom in limiting government (of whatever sort) to a least common denominator between the City of God and the City of Man; i.e. the protection of life, liberty and property. St. Augustine—along with St. Thomas, as noted above—believed vices must be tolerated because governmental suppression would only result in more evils:
What can be mentioned more sordid, more bereft of decency, or more full of turpitude than prostitutes, procurers, and the other pests of that sort? Remove prostitutes from human affairs, and you will unsettle everything because of lusts; place them in the position of matrons, and you will dishonor these latter by disgrace and ignominy. This class of people is, therefore, by its own mode of life most unchaste in its morals; by the law of order, it is most vile in social condition. [De Ordine, 2.4]
In condemning prostitution, St. Augustine makes it clear that just because we do not punish these sins does not mean we approve of the conduct. In his letter to Macedonius (413-14 A.D.), he uses the example of Jesus and the woman taken in adultery (whom Jesus saved from stoning). Augustine recommends that believers of his own day adopt Jesus’ merciful example: “Impious Jews yielded to his pronouncement; may pious Christians do so too.” He also notes that “[God] punishes very few offenses in this life, in case no one believes in divine providence; and he keeps most of them back for the final assessment, in order to remind us of that future judgment.” [“Letter 153: to Macedonius,” ¶ 4, 11]