I came across this recently, a Letter from James Madison to F.L. Schaeffer, dated 1821, in which the Father of the Constitution and the author of the Bill of Rights credits Luther with his doctrine of the Two Kingdoms for the American handling of church and state:
–I have received, with your letter of November 19th, the copy of your address at the ceremonial of laying the corner-stone of St Matthew's Church in New York.
It is a pleasing and persuasive example of pious zeal, united with pure benevolence and of a cordial attachment to a particular creed, untinctured with sectarian illiberality. It illustrates the excellence of a system which, by a due distinction, to which the genius and courage of Luther led the way, between what is due to Caesar and what is due God, best promotes the discharge of both obligations. The experience of the United States is a happy disproof of the error so long rooted in the unenlightened minds of well-meaning Christians, as well as in the corrupt hearts of persecuting usurpers, that without a legal incorporation of religious and civil polity, neither could be supported. A mutual independence is found most friendly to practical Religion, to social harmony, and to political prosperity.In return for your kind sentiments, I tender assurances of my estaeem and my best wishes.
Notice that this is not the “wall of separation” advocated by Jefferson, but a distinction in which both realms flourish as individual Christian citizens fulfill “both obligations” to both church and state.