Of God and His Creatures

The upshot of the foregoing chapter is this, that a man in mortal sin has got himself into a false position; and his position will go on growing morally worse unless he makes strenuous efforts to get out of it, turning to God and asking His aid: if he will not do that, he must be answerable for the growing deterioration of his state. He sins therefore, but it is not always easy to say precisely when he sins. In general, he sins when he could help himself to do better, and will not.

Of God and His Creatures: 3.161