An excerpt from a crucial section of my paper, “Natural Law and Which Two Kingdoms,” to be presented next weekend at the American Academy of Religion:

We must recognize that there were at least two sharply divergent conceptions of the “two kingdoms” that emerged from the sixteenth century, and, of course, a number of more or less consistent half-way houses between them.  Unsurprisingly, these different conceptions, and the way they used natural law, will undermine neat modern preconceptions about what natural law might be, and will suggest several different ways of applying it to a Christian society.  

Martin Luther offers a succinct statement of the first conception in 1521: “The kingdoms of the world are ruled by human laws which evidently have to do with things temporal; the kingdom of Christ is ruled by the pure and simple word of the Gospel.”  For the second, we have the Scottish Second Book of Discipline (1578): 

“The Kirke . . . hath a certaine power granted by God, according to the which it uses a proper jurisdiction and governement, exercised to the comfort of the whole Kirke.  The Policie of the Kirk flowing from this power, is an order or forme of spirituall government . . . different and distinct in its own nature from that power and policie, which is called civill power, and appertaineth to the civill government of the commonwealth: albeit they be both of God.”

Whereas Luther predicates two realms, one of law and jurisdiction, and another of pure grace and liberty, in Scotland we seem back to something akin to Gelasius and the medieval “two swords” doctrine: “two there are by whom this world is governed”–the civil and the ecclesiastical authorities.  Both have power, law, and jurisdiction under Christ, but they govern different functions.  For Luther, on the contrary, we find all power, law, and jurisdiction classed as part of the civil kingdom; Mosaic law, evangelical law, and natural law all fall on this side of the equation.  In short, the “spiritual kingdom” is the Church, but what we would call the “invisible Church,” though perhaps a better term would be the “evangelical Church” taking visible form only in the dynamic preaching of the Gospel and administration of the sacraments.  The visible, institutional Church, the gathered congregation that must be organized, ritualized, and governed, is part of the realm of “polity,” part of the sphere of human authority which it occupies in common with the more mundane concerns of the civil magistrate.  Indeed, the visible Church is simply the communion of the faithful, and as such, includes the civil magistrate if he be Christian, and his government, if the society be Christian.  The continuing “Christendom” idea, the corpus Christianorum, and the civil jurisdiction over the Church that usually went with it, is thus not some inconsistent holdover that Luther’s two-kingdoms theory has failed to exorcise, as VanDrunen suggests, but is part and parcel of it.  Human life is not a two-dimensional map onto which the two-kingdoms are drawn as a dividing line between spheres of jurisdiction, but rather, a two-dimensional map with which the civil kingdom is coterminous, and of which the spiritual kingdom might be said to form the third dimension–the vertical God-ward relation which animates all the rest.

What does this mean for natural law?  Well, for Luther, the contrast is not so much between natural law and divine law (Scripture) as between law and grace.  Scripture contains law too, and this is taken to be harmonious with the natural law, helping to govern the civil kingdom as illumination and application of natural law principles.  As much that we would call “religious” falls within the realm of the earthly kingdom, so it falls within the orbit of natural law, which cannot thus serve as the means for a thoroughgoing separation of church and state.  Not that Luther offers us a complete fusion of church and state–mindful of the intimate relationship between the outward ministry of the visible Church, and the inward power of the Gospel which breaks through it, Luther was wary of making the institutions of the Church simply a department of State (although not very successful in preventing it), and argued for the importance of maintaining three distinct “hierarchies” within the earthly kingdom–state, church, and family.