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.
After Aristotle 
It seems to me we’re somewhat at cross purposes.
The distinctions are important intellectually, and they help discussion with well-disposed people, but I doubt they can bear much weight as a practical matter.
If someone believes there’s a system of natural ends that gives rise to an obligatory natural morality, it seems to me he’ll likely end up believing there’s a authoritative person (God) whose ends those natural ends are. The whole picture seems more cohesive that way, and people settle into views that form a cohesive concrete picture. That’s how they carry on their lives.
Also, views on various sides of the matter affect each other. If someone’s a Hindu who thinks of God as ultimately impersonal, with no goals, then he’s likely to see the world as illusory or anyway not a meaningful order. If he’s an atheist who wants to exclude God, he’s likely to see the world as atoms bouncing off each other in the void, with no natural ends.
Even if he’s a theist what he believes about God and nature go together. Moslems (I am told) reject natural law, and their God is infinitely transcendent and knowable only in the form of arbitrary command. The two points seem to go together. Catholics believe in contrast that the world was created as intrinsically good, so much so that it lent itself to God becoming incarnate in it. They also have a strong sacramental principle. Those views seems to me closely connected with a view of nature as loaded with natural ends and meanings.
So if you decide what nature’s like it seems to me you’ve gone a long way toward deciding what God’s like, and conversely. That’s relevant to my piece in Crisis, because it means that natural law can’t support any very strong version of religious pluralism.
Thank you so much for your thoughtful comments.
I find your statement, “If someone believes there’s a system of natural ends that gives rise to an obligatory natural morality, it seems to me he’ll likely end up believing there’s a authoritative person (God) whose ends those natural ends are,” particularly interesting. There is, for one thing, the question of just what the obligatory and the forbidden would or could be in a natural law theory, particularly in an Aristotelian natural law theory. There is also the more intriguing question, at least for me now, of whether a strictly Aristotelian naturalism, say that of an Aristotle who (to indulge in an utterly impossible thought experiment) had the opportunity to learn all that he could from the study of Thomas Aquinas, without converting to Christian supernaturalism, would posit for purely rational reasons the existence of a personal God. A personal God would, of course, have to be a person, that is, an intelligent being and one loving and willing and, unlike that of the Aristotle who did not have the benefit of reading Aquinas, one the scope of whose knowledge and love and will included all reality in addition to himself or itself.
Stepping back from the abyss, you are surely right that one’s various views of nature, reason, God, tend to cohere; but not absolutely, for we are not always completely coherent.
One more note: I’ve done some study of Islamic philosophy and theology and while it is surely the case that most Muslim theologians, in particular those within the Sunni schools, tend towards strong versions of the voluntarism you point to, one will find even today an occasional admirer of the relatively rationalist Averroes, i.e., Ibn Rushd. Within Shiite Islam, especially in Iran, traditional philosophy is a major component of the training of clerics.