This chapter is complementary to Chapter CLVI. There we saw that a man in the state of grace still needs a succession of actual graces to enable him to observe the law of God on essential points steadily and to the end. Here we learn that a man in mortal sin needs actual graces to prevent his falling deeper. This chapter follows from the former as the less from the greater: for the victory that the man in grace cannot gain, still less can the sinner gain. The two chapters together argue that, left to his own unaided nature, unsupported by the movements of actual grace, every man capable of mortal sin will at times commit such sin.
Of God and His Creatures: 3.161