For liberals, the purpose of government is to promote excellence by maximizing the ability of each individual to develop his unique talents. For the most part, this is done by limiting the role of the state to the resolution of competing claims among citizens; however, the state also has a positive role to play, by providing education to everyone, and resources to those who would otherwise be deprived of the opportunity to thrive. Liberals believe that God does not exist, or that it is impossible to develop a consensus as to the nature of his will, so attempting to design a society that would make him happy is futile. Excellence is measured by the prosperity of the country, the quality of its culture, and its strength and influence internationally.
For people like Adrian Vermeule and William Barr, the purpose of government is to promote virtue, as defined in commonly-accepted religious texts. The virtuous society is one that is pleasing to God, and promotes the spiritual health of its citizens. As a result of the Fall, men are foolish, weak, and disposed to sin; the point of government is to use its power to force them to be virtuous. They may object in the short run, but in the long run, it is for their own good, and they will thank you for it.
It hardly needs to be said that the latter perspective has its intellectual roots in medieval Christianity. You may have believed that those ideas died when the religious wars in Europe ended inconclusively, but if you did, you were wrong.
The irony, of course, is that Barr’s chosen instrument for creating a virtuous America is a man who violates most of the Ten Commandments every day, and probably doesn’t even know what they are. Oh, well. The Lord, as they say, works in mysterious ways.