I'm not much familiar with the work of Harold Berman, the late Harvard Law Professor who just recently passed. He seems to have been a giant in the field of legal history, doing ground breaking work on the evolution of the common law. He also sympathized with the lowering the wall of separation view of the Constitution. And he did much work showing the influence of religion on law.
Though I approach these issues with more of a bias towards the secular side, a case can be made that religion, and yes, the Christian religion, has significantly influenced the civil law. However, the problem is those most notable for making such claims are clowns, not real scholars. Historian Mark Noll, one of the most distinguished scholars of the history of religion, and himself a conservative evangelical, wrote a book entitled The Scandal of the Evangelical Mind. The premise of the book is that it's a scandal that televangelists and pseudo scholars with mail order degrees like David Barton, Grant Jeffries, and Kent Hovid are the most prominently known faces of evangelical Christianity making public arguments. Roman Catholics don't seem to have this problem (although there are a few RC clowns). For instance, as a libertarian, I might disagree with the socially conservative politics of First Things magazine; but they keep their intellectual standards at the highest level (as far as I know, they've stayed away from the Robert Dornan, Alan Keyes types).
Berman became an evangelical Christian (I do believe) sometime late in his life. And though I would probably disagree on the emphasis he gave to religious sources in the development of law, from him you were likely to get a genuine scholarly, meticulously researched, and properly nuanced argument. (Something you don't get from the "resources" pushed by the televangelists, and used by the home schooled crowd.)
Though, that he coauthored a brief for the American Center For Law and Justice with Jay Sekulow defending the public display of the Ten Commandments in Texas gives me a little bit of pause. I completely understand the argument that such a display would be constitutional; one can plausibly argue that the First Amendment, properly understood, ought not forbid government from posting any religious messages, including "under Allah" or "under no God." However, whether the Ten Commandments is "foundational" to American (or Texas) law, what the brief in part argues, is highly debatable.
In fact, how the Ten Commandments relate to American law is instructive of how America's Founders and the Protestant and Enlightenment reformers they followed dramatically secularized the civil law. As Berman and a few others would note, arguments over "separation of Church and State" or what belongs to Ceaser and what belongs to God didn't begin with the Protestant Reformation or the Enlightenment. However, America's Founders did something dramatic: They posited an unalienable natural right to religious liberty, not just for orthodox Christians, but for all. And this in turn necessarily demanded the privatization of a great deal of -- but not all -- religiously based laws.
Previously, under the old order, the entire Ten Commandments were incorporated into the civil law. At least that's how it was in all American colonies except Rhode Island. For instance, the Massachusetts Body of Liberties states:
Deut. 13. 6, 10. Deut. 17. 2, 6. Ex. 22.20)
If any man after legall conviction shall have or worship any other god, but the lord god, he shall be put to death.
(Ex. 22. 18. Lev. 20. 27. Dut. 18. 10.)
If any man or woeman be a witch, (that is hath or consulteth with a familiar spirit,) They shall be put to death.
(Lev. 24. 15,16.)
If any person shall Blaspheme the name of god, the father, Sonne or Holie Ghost, with direct, expresse, presumptuous or high handed blasphemie, or shall curse god in the like manner, he shall be put to death.
Roger Williams' experiment in Rhode Island was the first that distinguished between the two tablets of the Ten Commandments. Certain commands are properly part of the civil law (don't steal, don't kill, etc.). Indeed, almost all other non-Judeo-Christian cultures also have these legal or social norms, arguably making them part of the "natural law" discovered by reason, that men of any or no religion can understand. Other commands absolutely must be consigned to the private sector. The first command, indeed the first four, properly have nothing to do civil law. Indeed, according to America's Founders, men have an unalienable natural right to break them. Men who, in Jefferson's words, worship no God or twenty Gods necessarily break the first four of the Ten Commandments.
All other key Founders from Washington to John Adams to Madison to Franklin, expressed similar sentiments. As Presidents Washington, Adams, and Madison themselves arguably broke these commands when they prayed to the Native American's pagan "Great Spirit" God by name. [Although they'd argue they genuinely believed all religions worshipped the same benevolent unitarian God.]
I've noted previously that I support publicly posting a statue of the Treaty of Tripoli which states "the Government of the United States of America is not, in any sense, founded on the Christian religion" next to the Ten Commandments wherever they would be displayed. Or perhaps we could, as proper symbolic gesture, post only the second tablet which contains the more secular oriented commands.
Harold Berman may have viewed things differently. That's fine. He'd played an important role in this scholarly dialogue. Christian Nation clowns like Barton, Federer, Kennedy et al. do not. Evangelicals will overcome their "scandal" when they value the genuine scholars among them like Noll, Berman, or Nathan Hatch, George Mardsen and Gary Scott Smith and ditch the clowns.