In his May 7, 2014, Crisis Magazine article, “Is Pluralism a Threat to Catholic Survival?” James Kalb tells his readers:
Natural law claims to be a philosophical position that can be developed and defended without regard to religion, so it does not seem to be religiously informed.
I will add that, if a theory or system of law which purports to be one of natural law cannot “be developed and defended without regard to religion,” then it is not a theory or system of natural law in any unambiguous sense of “natural,” at least of “natural” as opposed to “supernatural.”
Yet Kalb goes on to rather explicitly embrace a sense of “natural” which is very much ambiguous and very much seeming “to be religiously informed,” if only (barely) implicitly.
The project may have seemed more plausible than it was because of the ambiguous status of natural law. Philosophy is not neutral. Ideas about God, nature, and reason depend on each other, and it seems natural that classical natural law is mostly a Catholic tendency and those who come to accept it are likely eventually to become Catholic as well. So a natural law public philosophy might well be viewed as religiously informed, since it evidently has some connection to religion without being explicitly religious. Still, the religion to which it has a connection is specifically Catholic….
Now, though I am not aware of any sophisticated social science that has taken up the issue, I will grant that “classical natural law is mostly a Catholic tendency,” if that is taken to mean that most advocates of natural law are adherents to Catholicism. But those among the Catholic advocates of natural law who are scrupulously attendant to the classical distinction between that which, on the Catholic view, is of supernatural origin and that which is of natural origin will also be scrupulously attendant to the more specific distinction between law purportedly based at least in part in supernatural revelation and law based entirely in natural reason; they will see the theses of the latter as constituting but a proper subset of those of the former.
I might add in passing that I do not see that “those who come to accept it [classical natural law] are likely eventually to become Catholic as well,” unless there are factors other than that adherence to natural law at work.
The distinction between natural law theory in the strict sense of “natural” and an ambiguously “natural” natural law theory can, like all important and well-drawn theoretical distinctions, have significant practical import. A Catholic wishing to persuade his or her non-Catholic fellow citizens that, say, the death penalty or abortion is wrong is more like to succeed by presenting truly cogent arguments based entirely in natural law theory or reason, if such arguments there be, than by presenting arguments relying in part or in whole on the authority of scripture, Catholic tradition, or faith.