Sandefur pointed me to this article from Claremont entitled, The Christian Origins of the Law. Now, obviously as someone who believes that law and government are, properly, in principle, secular entities, I have a few comments.
First, what we know as "the law" is the product of thousands of years of evolving doctrine in the West. And because for much of that time, Church and State were thought of as one, obviously Christianity will have had some meaningful influence on the law. For instance, the article notes:
Melanchthon's method of analysis, set by the categories of the second table of the Decalogue, derived the basic topics of our contemporary law school curriculum, constitutional law, family law, criminal law, property law, etc. Berman does a great service to English readers in introducing the work of other important legal innovators, such as Johann Schwarzenberg in the field of criminal law, and Johann Apel and Konrad Lagus in the areas of civil, economic, and social law.
Let's unpack this: Why the "second table of the Decalogue"? Well, it's because that's the table that contains essentially secular cross-cultural norms! As Christopher Hitchens notes in this article:
There has never yet been any society, Confucian or Buddhist or Islamic, where the legal codes did not frown upon murder and theft. These offenses were certainly crimes in the Pharaonic Egypt from which the children of Israel had, if the story is to be believed, just escaped....
In much the same way, few if any courts in any recorded society have approved the idea of perjury, so the idea that witnesses should tell the truth can scarcely have required a divine spark in order to take root.
Now, I was unaware that in Europe in the middle ages, a serious attempt at legal distinction between the first and second tables of the Decalogue was made (I'm willing to look at the evidence; but off the bat, I'm highly dubious). I have studied in meticulous detail the evolution of Church and State law in American and English law. And from my knowledge, before Roger Williams made his utterly novel arguments about the need to separate Church and State and recognize religious liberty, Church and State were thought of as one and no distinction was made between the different tables of the Decalogue in terms of what in "the Christian religion" should also exist in the civil law.
As David Barton (unintentionally) comically writes in this brief submitted in one of the Ten Commandments cases:
In an effort to substantiate this position historically, critics often point to the Rhode Island Colony under Roger Williams and its lack of civil laws on the first four commandments to "prove" that American society was traditionally governed without the first "tablet." However, they fail to mention that the Rhode Island Colony was the only one of the thirteen colonies that did not have civil laws derived from the first four divine laws -the so-called first "tablet." Significantly, every other early American colony incorporated the entire Decalogue into its own civil code of laws.
Now, I realize that Barton is neither a credible nor a professional historian and thus could be wrong, as he often is in his factual assertions. But from what I've independently verified, Roger Williams's Rhode Island was the first colony which believed government should be secular in its essential functions and that it was thus not a good idea to attempt to write the Christian religion into the civil law. As Williams himself put it: "All civil states with their officers of justice, in their respective constitutions and administrations, are . . . essentially civil, and therefore not judges, governors, or defenders of the Spiritual, or Christian, State and worship. . . ." So it makes sense that he would be the first to make such a distinction between the two tables of the Decalogue.
[Listen to this excellent lecture by Martha Nussbaum on Roger Williams.]
The Claremont article asserts the influence of Puritan and Calvinist theory on the law:
Here, too, a breakdown in the settlement of the Peace of Augsburg, and its parallel developments in the rest of Europe, led to a new period of crisis, culminating in the Puritan-led "English Revolution."
If the contribution of the Lutheran reforms was an emphasis on positivism, on the territorial Christian prince as law-giver, Calvinist-inspired reforms gave the impetus to a robust sense of the historical development of law. Here, "the Calvinist doctrine that history is wholly within the Providence of God" underlay a conception of England's history and institutions as "the heritage on which their constitutional law was founded and which gave guidance for its future development." While in some sense the features of a distinctive English philosophy of law predated this period, with the 17th-century transformation, the historical common-law tradition came into focus, crystallized, and sprang to life, giving rise to the works of the great common-law jurists Coke, Selden, and Hale. Here, too, Calvinist covenant theology spilled over to shape the transforming, innovative initiatives of a rich and vibrant voluntary associational life, leading to communitarian joint-stock enterprises, reform of private and public finance, charity schools and poor relief, to name only some of the most significant legal contributions of the period.
But what was distinctive about the American Puritans and Calvin was that they drew no distinction between Church and State, between the secular and the sacred. Roger Williams's rejection of the "Christian Commonwealth," and his belief that government should be, in principle, secular got him banished from Massachusetts.
Further, let's look at some of those Puritan laws and see what it means to incorporate the entire Decalogue into civil law. As Barton's brief notes, incorporating the First Command in Puritan Massachusetts required the following: "If any man after legal conviction shall have or worship any other god but the Lord God, he shall be put to death. Deut. 13.6, 10, Deut. 17.2, 6, Ex. 22.20."
In this sense, "the Christian Religion" has been almost entirely and rightly purged from the law. Indeed, our liberal democratic Founding, with its unalienable rights of conscience, required such a purge.
Here is Walter Berns on Calvin's Geneva arguing that on Church and State matters, Calvin's "Christian Commonwealth" was the polar opposite of what the Declaration of Independence required:
For Calvin, liberty of conscience meant just that, and no more than that. If someone gave voice to his conscience, thus being heard or read by others, he might rightly be punished. So it was that, as the effective governor of his city of Geneva, Calvin had one of his anti-Trinitarian critics [Servetus] put to death. Berns, Making Patriots, p. 42.
Berns eloquently writes on how recognizing the unalienable rights of conscience, which rights by nature belong equally to the believer and heretic alike, required a dramatic realignment on Church/State matters in liberal states.
Liberty of conscience was widely accepted at the time of the Founding, but this did not prevent some jurists and legislators from insisting, at least for a while (and given our principles it could be only for a while), that Christianity was part of the law, meaning the common law. So it had been in England, and so, it was assumed by some (but not by Jefferson), it would continue to be in America. But there is no disagreement about the place of the common law. Indeed one of the first things done by the states after independence was to declare (here in the words of the New Jersey constitution of 1776) that "the common law of England, as well as so much of the statute law, as have been heretofore practiced in this Colony, shall remain in force, until they shall be altered by a future law of the Legislature; such parts only excepted, as are repugnant to the rights and privileges contained in this Charter [or constitution]."
But if the "rights and privileges" contained in the various state charters or constitutions included the right of liberty of conscience, and if, in turn, this right required, in Madison's words, "a perfect separation between ecclesiastical and civil matters," what did it mean to say that Christianity was part of the common law? Very little, as it turned out; and it turned out as it had to turn out. p. 33
In other words, liberty (and I should note "equality" too) of conscience for the believer and heretic alike required a purge of the sacred aspects of the Christian religion from the law, leaving behind the secular. And we are better off for it.