Pelagians asserted that God would not command man to do “what was impossible for human volition.” To this, St. Augustine (354-430) in Book II of his treatise De Peccatorum meritis et remissione (On the Merits and Remission of Sins) replies: “But they do not see, that in order to overcome certain things, which are the objects either of an evil desire or an ill-conceived fear, men need the strenuous efforts, and sometimes even all the energies, of the will. . . . The Lord, therefore, foreseeing that such would be our character, was pleased to provide and endow with efficacious virtue certain healthful remedies against the guilt and bonds even of sins committed after baptism,—for instance, the works of mercy,—as when he says: ‘Forgive, and ye shall be forgiven; give, and it shall be given unto you’ (Lk 6:37, 38).” (II, 3) Thus, God does not command the impossible, but neither does He command that we work without the assistance of His grace.
Furthermore, God wants us to do our part in cooperation with His grace. St. Augustine writes: “God is said to be ‘our Helper’ (Ps 40:17; Ps 70:5); but nobody can be helped who does not make some effort of his own accord. For God does not work our salvation in us as if he were working in insensate stones, or in creatures in whom nature has placed neither reason nor will.”
St. Augustine calls our attention to the Parable of the Pharisee and the Publican. The Pharisee in this story is a person who, by all outward appearances, is good and upright, and who, in his heart, gives God the glory due to Him by expressing his gratitude for the gifts he has received. Yet, he is found lacking. St. Augustine explains: “Although he erred in thinking that he needed no addition to his righteousness, and supposed himself to be saturated with abundance of it, he nevertheless gave thanks to God that he was not ‘like other men, unjust, extortioners, adulterers, or even as the publican; for he fasted twice in the week, he gave tithes of all that he possessed’ (Lk 18:11-12). He wished, indeed, for no addition to his own righteousness; but yet, by giving thanks to God, he confessed that all he had he had received from Him. Notwithstanding, he was not approved, both because he asked for no further food of righteousness, as if he were already filled, and because he arrogantly preferred himself to the publican, who was hungering and thirsting after righteousness.” (II, 6) One can only marvel at the value God places on the virtue of humilty.
Quotations from A Select Library of the Nicene and Post-Nicene Fathers of the Christian Church, Vol. V, ed. Philip Schaff (Edinburgh: T&T Clark, 1886).