This dogma can be convincingly demonstrated both from Sacred Scripture and Tradition.
(1) God Himself complains through the mouth of the prophet Isaias: “What is there that I ought to do more to my vineyard, that I have not done to it? Was it that I looked that it should bring forth grapes, and it hath brought forth wild grapes?”(109) This complaint clearly applies to the Jews. Yahweh did for the Jewish nation whatever it behooved Him to do lavishly (_gratia vere sufficiens_), but His kindness was unrequited (_gratia mere sufficiens_). In the Book of Proverbs He addresses the sinner in these terms: “I called, and you refused: I stretched out my hand, and there was none that regarded.”(110) What does this signify if not the complete sufficiency of grace? The proffered grace remained inefficacious simply because the sinner rejected it of his own free will. Upbraiding the wicked cities of Corozain and Bethsaida, our Lord exclaims: “If in Tyre and Sidon had been wrought the miracles that have been wrought in you, they had long ago done penance in sackcloth and ashes.”(111) The omniscient God-man here asserts the existence of graces which remained inefficacious in Corozain and Bethsaida, though had they been given to the inhabitants of Tyre and Sidon, they would have proved effective. The conclusion evidently is: these graces remained ineffective, not because they were unequal to the purpose for which they were conferred, but simply and solely because they were rejected by those whom God intended to benefit.(112)
(2) Though they did not employ the name, the Fathers were thoroughly familiar with the notion of sufficient grace.
Thus St. Irenaeus comments on our Lord’s lamentation over the fate of the Holy City: “When He says: (Matth. XXIII, 37): ‘How often would I have gathered together thy children, ... and thou wouldest not,’ He manifests the ancient liberty of man, because God hath made him free from the beginning.... For God does not employ force, but always has a good intention. And for this reason He gives good counsel to all.... And those who do it [_gratia efficax_] will receive glory and honor, because they have done good, though they were free not to do it; but those who do not do good will experience the just judgment of God, because they have not done good [_gratia inefficax_], though they were able to do it [_gratia vere et mere sufficiens_].”(113) St. Augustine is in perfect agreement with ecclesiastical tradition, and the Jansenists had no right whatever to claim him for their teaching. “The grace of God,” he expressly says in one place, “assists the will of men. If in any case men are not assisted by it, the reason lies with themselves, not God.”(114) And again: “No one is guilty because he has not received; but he who does not do what he ought to do, is truly guilty. It is his duty to act if he has received a free will and amply sufficient power to act.”(115)