Spealing of Calvin, Dr. Mattson also has a post in which he explains the problem with folks like Van Drunen trying to associate Calvin with their position (link to post). Mattson makes an excellent observation about the preface to Calvin's great Institutes of the Christian Religion:
Before Calvin ever gets to writing the book, he begins with something called a "Prefatory Address to King Francis I of France." It seems fairly strange that a man who believes Christian doctrine to be irrelevant to the "civil" realm would dedicate his work on Christian doctrine to the head of the civil realm. But Calvin is more specific. He writes: "For the Most Mighty and Illustrious Monarch, Francis, Most Christian King of the French, His Sovereign, John Calvin Craves Peace and Salvation in Christ." So... Francis is a "Christian" monarch. This way of speaking is anathema to modern Two Kingdoms advocates.There is more in Dr. Mattson's post - I encourage you to check it out.
When offering his defense to Francis, he writes: "Worthy indeed is this matter of your hearing, worthy of your cognizance, worthy of your royal throne! Indeed, this consideration makes a true king: to recognize himself a minister of God in governing his kingdom. Now, that king who in ruling over his realm does not serve God's glory exercises not kingly rule but brigandage."