Folks who are libertarians fundamentally do not believe in natural authority... If you do believe in natural authority, the libertarian position looks insane.
Now, I'd classify myself, politically, as a libertarian. Despite this, I do very much believe in natural authority, in the Romans 13 sense - "1. Let every soul be subject unto the higher powers. For there is no power but of God: the powers that be are ordained of God."
I very much suspect that the two positions are not mutually exclusive, though I'm pretty sure that if I fumbled around and tried to explain it properly, I'd just end up sounding like a blithering idiot.
And yet, I'll try.
Aretae restates his argument as "There are exactly two interactions in the world... freely chosen and thug-based." In terms of the Christian faith, so long as you steer clear of the Calvinistic view of election, then I think you can argue that submission to the authority of God on the part of a believer is entirely consensual - consider Romans 12, for example.
Now, granted - it is very much a "freely chosen" interaction on our part only because God has allowed it. The fact remains, though, that even though there is no comparison between myself and the creator of the universe - this is not at all a "freely chosen" interaction between like parties! - it is still a choice. The essential question of Christianity is that God Himself, in eternity past, made the choice to die for my sins. How I freely choose to react to that action on the part of God is my choice alone.
More later, I'm sure. Suffice it to say that, at least at a first glance, I don't consider Christianity and libertarian politics to be necessarily exclusive. As such, I'm interested to see where Aretae will take his initial argument.