Chapter 29 [XXVI.]—A Simile to Show that God’s Grace is Necessary for Doing Any Good Work Whatever. God Never Forsakes the Justified Man If He Be Not Himself Forsaken. 1203
Observe, indeed, how cautiously he expresses himself: “God, no doubt, applies His mercy even to this office, whenever it is necessary because man after sin requires help in this way, not because God wished there should be a cause for such necessity.” Do you not see how he does not say that God’s grace is necessary to prevent us from sinning, but because we have sinned? Then he adds: “But just in the same way it is the duty of a physician to be ready to cure a man who is already wounded; although he ought not to wish for a man who is sound to be wounded.” Now, if this simile suits the subject of which we are treating, human nature is certainly incapable of receiving a wound from sin, inasmuch as sin is not a substance. As therefore, for example’s sake, a man who is lamed by a wound is cured in order that his step for the future may be direct and strong, its past infirmity being healed, so does the Heavenly Physician cure our maladies, not only that they may cease any longer to exist, but in order that we may ever afterwards be able to walk aright,—to which we should be unequal, even after our healing, except by His continued help. For after a medical man has administered a cure, in order that the patient may be afterwards duly nourished with bodily elements and ailments, for the completion and continuance of the said cure by suitable means and help, he commends him to God’s good care, who bestows these aids on all who live in the flesh, and from whom proceeded even those means which [the physician] applied during the process of the cure. For it is not out of any resources which he has himself created that the medical man effects any cure, but out of the resources of Him who creates all things which are required by the whole and by the sick. God, however, whenever He—through “the one mediator between God and men, the man Christ Jesus”—spiritually heals the sick or raises the dead, that is, justifies the ungodly, and when He has brought him to perfect health, in other words, to the fulness of life and righteousness, does not forsake, if He is not forsaken, in order that life may be passed in constant piety and righteousness. For, just as the eye of the body, even when completely sound, is unable to see unless aided by the brightness of light, so also man, even when most fully justified, is unable to lead a holy life, if he be not divinely assisted by the eternal light of righteousness. God, therefore, heals us not only that He may blot out the sin which we have committed, but, furthermore, that He may enable us even to avoid sinning.
See the treatise De Peccatorum Meritis, ii. 22.