… is from page 31 of Randy Barnett’s and Evan Bernick’s important 2021 book, The Original Meaning of the Fourteenth Amendment: Its Letter and Spirit (original emphasis):
We concur with the claim that the “due process of law” is a “procedural” guarantee. But we then show that this procedure is “substantive” insofar as it requires judges to examine the substance of legislation. When it comes to the legitimacy of statutes, the “due process of law” is not limited to whether a statute was duly enacted by the requisite legislative procedure.
The original meaning of “due process of law” in the Fourteenth Amendment guarantees some judicial process before any person can be deprived of life, liberty, or property. This judicial process includes a jury trial. The question then concerns the proper scope of this process. Such a judicial process potentially involves an inquiry into two questions: (1) Was an accused person actually guilty of violating a preexisting law – whether a statute or the common law, and (2) if a statute was being enforced, was it within the proper power of the relevant legislature to enact? The second of these questions requires an examination of the substance of the statute that is being enforced.