The past few months I've been working hard to understand the issue of nature/grace and the Reformed understanding of this "vs. the Catholic understanding". I put that in quotes because a large part of figuring this out involves trying to figure out whether the Catholic understanding is substantially different from the Reformed understanding. Sources conflict on exactly what is going on. Some contend that the Reformers broke away from a Catholic understanding that rested on Thomas Aquinas and was a faulty view of grace that ended up divinizing human nature, destroying it in the process (so Bavinck). At the same time, when personally reading Aquinas, he seems to be teaching the standard Reformed view of nature/grace found in Turretin and Junius for example. So what's the deal? Ultimately what I believe happened can be summarized here and here. (see also here and here)It seems to me that Aquinas' view of nature/grace just is the Reformed view of it, however, later Catholic theologians misunderstood Thomas and took his supposed view in a bad direction which was opposed by later Reformed theologians such as Bavinck.
I think this is why Richard Muller in his Dictionary of Latin and Greek Theological Terms in defining the donum superadditum states that,
Aquinas maintained that the donum superadditum was part of the original constitution of man and that its loss was the loss of the original capacity for righteousness. Since the superadded grace was not merited in the beginning, it cannot be regained by merit after the fall.
Then in defining the Reformed counter donum concreatum states,
The Protestant argument was that the donum gratuitum, the utterly free gift, of iustitia originalis(q.v.) was part of the original constitution of man and therefore a donum concreatum, naturale, or intrinsecum rather than something superadded to the original constitution of man. By extension, the loss of the iustitia originalis in the fall was the loss of something fundamental to the constitution of man that could be resupplied only be a divine act and not, as the semi-Pelagian tendency in late medieval Scotism and nominalism indicated, something superadded that could be regained by a minimal act of human obedience.
Notice the similarity between Aquinas and the Reformers here: original righteousness is part of the original constitution of man and cannot be regained by human obedience after the fall, but only by the grace of God. Yet Muller distinguishes the two in saying that the Reformed conception did not see original righteousness as something superadded to the original constitution of man. I believe this is done because "superadded" came to mean something different to later Roman Catholic theologians than it did to Aquinas (see my comments on "pure nature" near the bottom), but though they did not use the same term as Aquinas (given that by their time, "superadded" meant something different), the Reformers affirmed in essence the same thing as he did, namely, that original righteousness, though a part of man's original constitution, is something accidental to him.
Another related issue I wish to bring up: I am not totally aware of Feser's theology l but I am sure Aquinas himself held to it, but we must remember too that later Catholic theologians, whom Turretin opposed, taught that Christ's soul had beatific knowledge of God in this life. This must be rejected. As I have said, beatific knowledge will occur only in heaven. Only the glorified state of man can partake of it. Turretin also opposed the idea of human beings existing in a state of pure nature, however, I do not think he meant the same thing as Feser does. When Turretin opposes pure nature he is fighting against the later Roman Catholic idea that man was not created with original righteousness, to which the Reformed countered that man was created good and upright with wisdom and holiness. I have found passages in both Augustine and Aquinas where they rather clearly seem to side with the Reformed on this issue against the Catholics that Reformers like Turretin were arguing against (for example "On the Trinity", Book XIII, Chapter 18 where Augustine opposes prelapsarian concupiscence [ Edit: see here also https://calvinistinternational.com/2018/06/28/concupiscence-sin-gay-christianity-desire-orientation/ ] and, more inferentially, Aquinas' Summa, Part III, Q. 41, Reply to Obj. 3, where Aquinas implies that concupisence is itself sin, which is contrary to modern Rome).
By the way, I also believe that something similar happened with Aquinas' view on transubstantiation. His view certainly isn't biblical but it also does not appear to be the same view that the Reformers were opposing. Aquinas did not hold to a corporeal presence. Later Roman Catholic writers, misunderstanding Aquinas (which is quite easy to do at this point) took him to be teaching that the eucharist literally became the corporeal body of Christ, which the Reformers rightly opposed. A philosophical critique of Aquinas' position is found here (a Biblical critique is found in any exegetical commentary on the Gospels/1 Corinthians 😉), note that in the 10th footnote, it is said that Turretin misunderstood the Romanists. Again, I believe this to be a failure to distinguish between Aquinas' view and the later Roman view. Turretin was opposing the Roman view of his time and did not misunderstand it, while at the same time marshaling arguments against it that still applied to Aquinas (so footnote 2).
