Carl Schmitt, Political Theology: Chapter 3, "Political Theology"

36: "All significant concepts of the modern theory of the state are secularized theological concepts, not only because of their historical development -- in which they were transferred from theology to the theory of the state, whereby, for example, the omnipotent God became the omnipotent lawgiver -- but also because of their systematic structure, the recognition of which is necessary for a sociological consideration of these concepts. The exception in jurisprudence is analogous to the miracle in theology."

Schmitt does more to justify the affinity of structure than the genealogy. He certainly shows that early modern political thought is saturated with theistic analogies. Schmitt insists that this is more than a historical accident. He thinks that the image of an omnipotent authority is essential to the conception of the state; it is required because law itself must be applied, and requires an authority to do the job. Moreover, he criticizes recent sociologically oriented analysis of the state for attempting to evade discussion of such a unitary personal authority.

In the end, however, he shortchanges the promise to demonstrate the derivation of concepts of sovereignty from theology. He falls back on arguing for the concurrent development of modern conceptions of God and sovereignty.

So he sees a parallel in early modern views of a God who established natural laws and then remained thereafter detached from creation, and of the sovereign as a lawgiver. (This is clearly Schmitt's preferred formulation, but the analogy here seems not too exact -- Schmitt's point about sovereignty is that it required an authority which can never really remain detached, after all.)

He sees an analogous development towards immanence in both fields in the 19th century -- God either doesn't exist or is simply identified with His concrete expression in the world; the sovereign doesn't exist or is identified with the the law or the actions of the people.

Along the way, he notes in passing (see 42 and 52) that legal reasoning has a form which is drawn from medieval thought -- which he finds natural because he holds that medieval thought was juristic rather than scientific in form. But, for me, this raises a question: from where did medieval thought acquire this juristic form? This reveals the problem with holding that modern concepts of the state are secularized borrowings from theology. Law and state didn't emerge for the first time in modernity. They were already present in classical times, and medieval thought took shape against this background. So Modern political thought may seem to have derived its concept of sovereignty from medieval theology only because the horizon is placed at the sixteenth century. In a longer view, the God of medieval thought may already have been shaped by ancient law.