In part you understood me correctly, in part not. First, God is the source of all moral norms, and that natural law describes the decent and moral life regardless of whether the person living that life accepts or rejects God. Consequently, someone who rejects God but wishes to live a decent and moral life does so by "borrowing" the natural law already laid down by God. So in the sense of the secular world, it is possible to live what is perceived by the secular world to be a decent and moral life according to the world's perspective. Christians use the example of the Jewish Pharisees, who in the sense of the world were righteous but in pursuing their sense of righteousness had lost sight of God. It is not possible to live a moral life according to God's perspective, unless a person has accepted God as his or her moral sovereign. In other words, without an outside sovereign (and this could mean either God or the state), human beings are incapable of creating consistent, long-term, moral judgments. Instead, we end up with people unable to distinguish between right and wrong. If you need proof, look to Peter Singer at Princeton, with his human ethic of baby slaying. I would also submit that even the state is incapable of enforcing any kind of objectively just, consistent morality long term because it is fundamentally subject to capture by non-moral people.
Second, as the article asserted, however, libertarianism only works in a community that shares a similar moral compass. That moral compass - whether God-given or created by people - can't be forced on someone, so the only means of achieving such a community is to keep it small. The larger the community, the greater the need to enforce what is perceived as God's law by imposing that law on people at the fringes. In the end, you end up with a human theocracy rather than a libertarian state.
Third, I'm pretty sure that the quote could be expanded to include societal morals and standards. And I'll even submit that there are individuals who - through reason and self-control - have adopted worldly standards of just and decent behavior while rejecting God and His laws. But the problem is that without an objective standard, it is impossible to maintain purely worldly morals. Once you open the door for relativist, non-objective morality then shame and moral opprobrium lose their ability to control behavior a priori, which in turn opens the door for the state to step in and control behavior post hoc.
Yes.
I've been trying to put that into words for quite a while. Thanks.