Still it is insisted, that exhortations are vain, warnings superfluous, and rebukes absurd, if the sinner possesses not the power to obey. When similar objections were urged against Augustine, he was obliged to write his book, De Correptione et Gratia, where he has fully disposed of them. The substance of his answer to his opponents is this: “O, man! learn from the precept what you ought to do; learn from correction, that it is your own fault you have not the power; and learn in prayer, whence it is that you may receive the power.” Very similar is the argument of his book, De Spiritu et Litera, in which he shows that God does not measure the precepts of his law by human strength, but, after ordering what is right, freely bestows on his elect the power of fulfilling it. The subject, indeed, does not require a long discussion. For we are not singular in our doctrine, but have Christ and all his apostles with us. Let our opponents, then, consider how they are to come off victorious in a contest which they wage with such antagonists. Christ declares, “without me ye can do nothing,” (John 15: 5) Does he the less censure and chastise those who, without him, did wickedly? Does he the less exhort every man to be intent on good works? How severely does Paul inveigh against the Corinthians for want of charity, (1Co 3: 3) and yet at the same time, he prays that charity may be given them by the Lord. In the Epistle to the Romans, he declares that “it is not of him that willeth, nor of him that runneth, but of God that showeth mercy,” (Rom 9: 16) Still he ceases not to warn, exhort, and rebuke them. Why then do they not expostulate with God for making sport with men, by demanding of them things which he alone can give, and chastising them for faults committed through want of his grace? Why do they not admonish Paul to spare those who have it not in their power to will or to run, unless the mercy of God, which has forsaken them, precede? As if the doctrine were not founded on the strongest reason – reason which no serious inquirer can fail to perceive. The extent to which doctrine, and exhortation, and rebuke, are in themselves able to change the mind, is indicated by Paul when he says, “Neither is he that planteth any thing, neither he that watereth; but God that giveth the increase,” (1Co 3: 7) in like manner, we see that Moses delivers the precepts of the Law under a heavy sanction, and that the prophets strongly urge and threaten transgressors though they at the same time confess, that men are wise only when an understanding heart is given them; that it is the proper work of God to circumcise the heart, and to change it from stone into flesh; to write his law on their inward parts; in short, to renew souls so as to give efficacy to doctrine

Institutes, Book 2, Chapter 5

Translating it from the Latin, Augustine’s book is entitled either On Admonition and Grace or On Rebuke and Grace. It is widely available on the web.

Calvin is right to think of it so highly.