The idea as I understand it is basically this:
- By nature people have the capacity to do certain kinds of good things: build houses, run countries justly, love their children, etc. However, none of this will lead to salvation. It doesn’t merit anything from God. By itself, it doesn’t make you worthy of eternal life.
- Grace, which is a free gift from God, “elevates” the soul so that it can share in eternal life. This enables “meritorious” good works which are worthy of eternal life. Grace comes through the sacraments.
The criticism is that this neat division between nature and grace, natural and supernatural, contributes to secularization because it leaves the world of “nature” to go on by itself, reserving grace/the supernatural for those things having to do with the world to come.
So, for instance, a student at the evangelical college where I used to teach, who was at that time a convinced and fairly radical libertarian, told me that he was working on a paper using Suarez and other late scholastics and talking about natural law. It transpired that he thought natural law was a “secular” concept that didn’t refer to God. I said, “I don’t know what Suarez thought, but I know that’s not what Aquinas thought.” I still don’t know if he or his secondary source was misreading Suarez or if Suarez really did distort Aquinas in this way. I don’t know even Cajetan’s work on Aquinas very well, let alone the later guys. But this is the sort of thing people who criticize the “two-tier” approach worry about. (There are other worries, too, but this is one relevant example.)
A more specific application of this problem concerns abortion. Catholics and other Christians are often criticized for “imposing their religious views” with regard to abortion. Catholics reasonably respond that murder is forbidden by natural law, and that we can discern by reason that the fetus is in fact a human being. But the pro-choice person is likely to respond: “So why is it that all the people who are anti-abortion just happen to be conservative religious people?” Now of course this isn’t true–there is a rising atheist prolife movement. But certainly being a traditional Christian makes you much more likely to be prolife, and it’s disingenuous for Catholics to say that their Catholicism has nothing to do with their being prolife. It pretty clearly does. But this is only a count “against” the claim that prolife arguments are based on natural law if we assume that natural law is supposed to be secular (and further that only secular-based arguments count “in the public square,” which is of course nonsense and almost never upheld consistently by anyone).
Edwin