One of the reasons theonomy failed as a movement, and this reason perhaps dovetails with why theonomy went Federal Vision and also failed to work out a coherent alternative, is that theonomists generally did not read the Protestant Scholastic sources carefully, to the degree they read them at all. This is not entirely theonomy’s fault. Reformed publishers have tone a woefully terrible job at making these (life-and-death important) sources available (yes, Baker Academic, I am talking about you!).
Nevertheless, some sources are available and Theonomists should have availed themselves of that. That raises another problem: reading these sources required reading these sources on the sources’ terms. Theonomists usually viewed anyone who disagreed with them as a “natural law adherent,” defining natural law as a mix of Locke, Newton, and Aquinas. Here is an experiment for you: pick up a theonomic text and find a fair definition of natural law on Reformed terms. Bahnsen avoids it in TiCE (though to be fair to Bahnsen, he never really opposed natural law). Gary North slams it but never really defines (or explains how modern Reformed accept natural law). The real villain, I think, is Kuyperianism (though, ironically, Kuyper himself was a pluralist). The result was the no-neutrality concept was applied to areas which really didn’t make sense in a practical way (yes, we should do math and plumbing to the glory of God, but there really isn’t a Christian praxis to Christian plumbing).
If you read Reformed natural law sources carefully, you will note that 1) they don’t necessarily contradict Moses [many advocated using the Mosaic judicials because of the wisdom found therein; as to what kind of theory they employed for which judicials were to be used is anybody’s guess], 2) they aren’t using the term “nature” to mean butterflies and puppies [which is how I had usually glossed it], and modern advocates of natural law theory even concede that theonomists were correct to raise a lot of these issues.
Secondly, and perhaps more importantly, reading the Protestant Scholastic sources on their own terms will also bring the reader face-to-face with their teachings on covenant and justification, areas which modern theonomists are painfully weak. For all of my previous criticisms of van Drunen and RS Clark, which I have now retracted, it is interesting to note that these guys adamantly insisted on the Protestant Scholastic teaching on natural law as thoroughly as said teaching on covenant theology. The two seemed to go together (I don’t think there is a 1:1 correlation, but a lot of people have speculated on the Federal implications of both Covenant Theology and federal politics ala Althusius).