For whatever this is worth. After fully rejecting theonomy/theocracy as a ground for the current political order, I spent many years trying to answer the question "on what basis, then, can we make any laws?" I've come to the place where I believe the Reformers (in general) and the Classical Liberals (in general) were correct. There is a Divine Lawgiver who has revealed through Natural Law and through conscience what is generally required of people--that these requirements flow from the "second table" of the moral law, and that he has revealed this law sufficiently enough even for unbelievers to recognize it. I found further support for this in the "two kindgoms" model, specifically in God's ordaining retributive justice as the norm for the common kingdom (Genesis 9:6). This is also why I am a libertarian--the non-aggression principle aligns rather nicely with the prohibitions of the 6th through 9th commandments (don't murder, don't steal, honesty in contracts, etc.). For a deeper study on this stuff, I'd highly recommend VanDrunen's three book series as well as Stephen Graybill's on the Natural Law tradition in the Reformed tradition. Let me know if you want the full titles. Well worth the time if you're interested in how Christians ought to think about, and to what end we ought to work, in the political order...