Summa contra Gentiles3.156 Chapter CLVI. That he who falls away from grace by sin, can recover again by means of grace

Hence it may be shown that by the assistance of grace, even when a man has not persevered, and has fallen into sin, he can be restored to good.

For it belongs to the same force to maintain a man’s health, and to mend it when broken: thus health is maintained in the body by the forces of nature, and by the same forces is restored when it is impaired. Now, as we have shown, man perseveres in good by the aid of divine grace. Therefore, if he has fallen through sin, he can be restored by the same assistance of grace.

Again. An agent that requires no disposition in the subject, is able to produce its effect in a subject no matter how disposed: and for this reason God, who in His action requires no disposition in the subject, is able to produce a natural form in a subject, without that subject being thereto disposed: as when He gives sight to the blind, and life to the dead, and so forth. Moreover, as He requires no natural disposition in a corporeal subject, so neither does He require merit in the will, in order to bestow grace, since this is given apart from merit, as we have proved. Therefore God is able to give man sanctifying grace, whereby sins are taken away, even after his fall from grace by sin.

Further. Those things alone is man unable to recover after their loss, which he acquires in birth, such as his natural powers and limbs: because man cannot be bom again. Now, the aid of grace is given to man not in birth, but when he is already in existence. Therefore after losing grace through sin, he can regain it so that his sins be wiped out.

Moreover. Grace is an habitual disposition in the soul, as we have shown. But habits acquired by means of acts, if they be lost, can be acquired anew by means of the acts whereby they were acquired. Much more, therefore, if we lose the grace which unites us to God and frees us from sin, can we regain it by the divine operation.

Moreover. In God’s works, as in nature’s, nothing is without a purpose; for even nature owes this to God. Now it would be useless for a thing to be moved, unless it were able to reach the end of the movement. Hence that which has a natural aptitude for being moved towards a certain end, must needs be able to reach that end. Now, after man has fallen into sin, as long as he remains in this state of life, he retains the aptitude for being moved to good: this is indicated by his desire for good, and his grief for evil, which remain in him after he has sinned. Therefore after he has sinned it is possible for man to return once more to good: and this is the effect of grace in man.

Further. In all nature there is not to be found a passive potentiality that is not reducible to actuality by some natural active power. Much less therefore is there in the human soul a potential that cannot be brought to act by the active power of God. Now, even after sin, the human soul retains the potentiality for good: because sin does not deprive the soul of its natural powers whereby it is ordered to its good. Therefore, by God’s power it can be restored to good, and consequently, by the aid of grace, man can receive forgiveness of his sins. Hence it is said (Isa. i. 18): If your sins be as scarlet, they shall be made white as snow: and (Prov. x. 12): Charity covereth all sins. This also we daily ask not in vain of the Lord, when we say: Forgive us our trespasses.

Hereby we refute the error of the Novatians who said that a man cannot obtain forgiveness for the sins committed by him after Baptism.