(CG 3/156). That Man needs the Assistance of Divine Grace to persevere in ... (Book 3. God, the End of Creatures) (Summa Contra Gentiles) (Aquinas, Thomas, St.)

IT belongs to the same power to continue and to repair after interruption, as is the case with the powers of nature in regard of bodily health. But man perseveres in good by the aid of divine grace: therefore, if he has fallen by sin, he may be recovered by help of the same grace.

2. An agent that requires no predisposition of its subject, can imprint its effect on its subject, howsoever disposed. But God, requiring no predisposition of the subject of His action, when the subject is corporeal,�as when He gives sight to the blind, or raises the dead to life,�does not require any previous merit either in the will for the conferring of His grace, which is given without merits (Chap. CXLIX). Therefore even after a man has fallen from grace by sin, God can confer on him the grace that puts the recipient in the state of grace, whereby sins are taken away.

5. In the works of God there is nothing in vain, as neither in the works of nature, for nature has this prerogative of God. Now it would be in vain for anything to move with no chance of arriving at its term. Whatever naturally moves to a certain end, must be somehow competent to get there. But after a man has fallen into sin, so long as the state of this life lasts, there remains in him an aptitude of being moved to good, shown by such signs as desire of good and grief at evil. Therefore there is some possibility of his return to good.

6. There exists in nature no potentiality, which cannot be reduced to act by some natural active power. Much less is there in the human soul any potentiality, which is not reducible to act by the active power of God. But even after sin there still remains in the human soul a potentiality of good, because the natural powers, whereby the soul is related to its proper good, are not taken away by sin.

Therefore it is said: Though your sins be as scarlet, they shall be made white as Snow (Isai. i, 18): Charity covereth all sins (Prov. x, 12). Nor do we ask of the Lord in vain, Forgive us our trespasses.

Hereby is refuted the error of the Novatians, who said that man cannot obtain pardon for sins committed after baptism.