For Aquinas, not only does everything depend upon the divine will, as Tolkien puts it, “in every detail and moment,” but he also holds that this absolute control represents not a threat to creaturely freedom, but a necessary condition for it.
At the most general level this is true because God is the concurrent source of the finite will’s existence, and therefore of its freedom. Secondly and more particularly, because God has created the will as free, when he acts on the will he does so in a way consistent with the nature he himself has given to it. In a passage from question six of his Disputed Questions On Evil, Thomas argues that, as the ultimate cause of all things, divine providence always acts in a way consistent with the manner of existence of each thing, so that God efficaciously acts on the individual free will, yet in a way that leaves, indeed, preserves, the will in its freedom. (This, incidentally, is why it is problematic to think of miracles, as many moderns do, in terms of divine “violation” of the laws of nature: for grace, or the “supernatural,” never “violates” nature, but always restores, redeems, and so “complements” her.) In his commentary on this passage by Aquinas, Brian Davies observes that human beings “are not free in spite of God, but because of God… [H]uman freedom is compatible with providence because only by virtue of providence is there any human freedom.”
It is precisely this Thomistic relationship between divine providence and free will, as I have been suggesting, that is observed in Tolkien, and as more than one of his commentators have recognized. Peter Kreeft, for example, comments how for Tolkien “divine predestination preserves human free will, because God invented it. As Aquinas says, man is free because God is all-powerful. For God not only gets everything done that He designs, but also gets everything done in the right way: subhuman things happen unfreely, and human things happen freely” (Kreeft, The Philosophy of Tolkien, 64). Similarly, Brad Birzer, in his article on Aquinas in the J.R.R.T. Encyclopedia, writes that “Aquinas argued that… free will could only be understood within God’s sovereignty and predestination.”