Objection 1. It would seem that man can rise from sin without the help of grace. For what is presupposed to grace, takes place without grace. But to rise to sin is presupposed to the enlightenment of grace; since it is written (Ephesians 5:14): "Arise from the dead and Christ shall enlighten thee." Therefore man can rise from sin without grace.
Objection 2. Further, sin is opposed to virtue as illness to health, as stated above (71, 1, ad 3). Now, man, by force of his nature, can rise from illness to health, without the external help of medicine, since there still remains in him the principle of life, from which the natural operation proceeds. Hence it seems that, with equal reason, man may be restored by himself, and return from the state of sin to the state of justice without the help of external grace.
Objection 3. Further, every natural thing can return by itself to the act befitting its nature, as hot water returns by itself to its natural coldness, and a stone cast upwards returns by itself to its natural movement. Now a sin is an act against nature, as is clear from Damascene (De Fide Orth. ii, 30). Hence it seems that man by himself can return from sin to the state of justice.
On the contrary, The Apostle says (Galatians 2:21; Cf. Galatians 3:21): "For if there had been a law given which could give life--then Christ died in vain," i.e. to no purpose. Hence with equal reason, if man has a nature, whereby he can he justified, "Christ died in vain," i.e. to no purpose. But this cannot fittingly be said. Therefore by himself he cannot be justified, i.e. he cannot return from a state of sin to a state of justice.
I answer that, Man by himself can no wise rise from sin without the help of grace. For since sin is transient as to the act and abiding in its guilt, as stated above (Question 87, Article 6), to rise from sin is not the same as to cease the act of sin; but to rise from sin means that man has restored to him what he lost by sinning. Now man incurs a triple loss by sinning, as was clearly shown above (85, 1; 86, 1; 87, 1), viz. stain, corruption of natural good, and debt of punishment. He incurs a stain, inasmuch as he forfeits the lustre of grace through the deformity of sin. Natural good is corrupted, inasmuch as man's nature is disordered by man's will not being subject to God's; and this order being overthrown, the consequence is that the whole nature of sinful man remains disordered. Lastly, there is the debt of punishment, inasmuch as by sinning man deserves everlasting damnation.
Now it is manifest that none of these three can be restored except by God. For since the lustre of grace springs from the shedding of Divine light, this lustre cannot be brought back, except God sheds His light anew: hence a habitual gift is necessary, and this is the light of grace. Likewise, the order of nature can only be restored, i.e. man's will can only be subject to God when God draws man's will to Himself, as stated above (Article 6). So, too, the guilt of eternal punishment can be remitted by God alone, against Whom the offense was committed and Who is man's Judge. And thus in order that man rise from sin there is required the help of grace, both as regards a habitual gift, and as regards the internal motion of God.
Reply to Objection 1. To man is bidden that which pertains to the act of free-will, as this act is required in order that man should rise from sin. Hence when it is said, "Arise, and Christ shall enlighten thee," we are not to think that the complete rising from sin precedes the enlightenment of grace; but that when man by his free-will, moved by God, strives to rise from sin, he receives the light of justifying grace.
Reply to Objection 2. The natural reason is not the sufficient principle of the health that is in man by justifying grace. This principle is grace which is taken away by sin. Hence man cannot be restored by himself; but he requires the light of grace to be poured upon him anew, as if the soul were infused into a dead body for its resurrection.
Reply to Objection 3. When nature is perfect, it can be restored by itself to its befitting and proportionate condition; but without exterior help it cannot be restored to what surpasses its measure. And thus human nature undone by reason of the act of sin, remains no longer perfect, but corrupted, as stated above (Article 85); nor can it be restored, by itself, to its connatural good, much less to the supernatural good of justice.
Source: Summa Theologica (New Advent)