Objection 1. It would seem that without grace man can avoid sin. Because "no one sins in what he cannot avoid," as Augustine says (De Duab. Anim. x, xi; De Libero Arbit. iii, 18). Hence if a man in mortal sin cannot avoid sin, it would seem that in sinning he does not sin, which is impossible.
Objection 2. Further, men are corrected that they may not sin. If therefore a man in mortal sin cannot avoid sin, correction would seem to be given to no purpose; which is absurd.
Objection 3. Further, it is written (Sirach 15:18): "Before man is life and death, good and evil; that which he shall choose shall be given him." But by sinning no one ceases to be a man. Hence it is still in his power to choose good or evil; and thus man can avoid sin without grace.
On the contrary, Augustine says (De Perfect Just. xxi): "Whoever denies that we ought to say the prayer 'Lead us not into temptation' (and they deny it who maintain that the help of God's grace is not necessary to man for salvation, but that the gift of the law is enough for the human will) ought without doubt to be removed beyond all hearing, and to be anathematized by the tongues of all."
I answer that, We may speak of man in two ways: first, in the state of perfect nature; secondly, in the state of corrupted nature. Now in the state of perfect nature, man, without habitual grace, could avoid sinning either mortally or venially; since to sin is nothing else than to stray from what is according to our nature--and in the state of perfect nature man could avoid this. Nevertheless he could not have done it without God's help to uphold him in good, since if this had been withdrawn, even his nature would have fallen back into nothingness.
But in the state of corrupt nature man needs grace to heal his nature in order that he may entirely abstain from sin. And in the present life this healing is wrought in the mind--the carnal appetite being not yet restored. Hence the Apostle (Romans 7:25) says in the person of one who is restored: "I myself, with the mind, serve the law of God, but with the flesh, the law of sin." And in this state man can abstain from all mortal sin, which takes its stand in his reason, as stated above (Question 74, Article 5); but man cannot abstain from all venial sin on account of the corruption of his lower appetite of sensuality. For man can, indeed, repress each of its movements (and hence they are sinful and voluntary), but not all, because whilst he is resisting one, another may arise, and also because the reason is always alert to avoid these movements, as was said above (74, 3, ad 2).
So, too, before man's reason, wherein is mortal sin, is restored by justifying grace, he can avoid each mortal sin, and for a time, since it is not necessary that he should be always actually sinning. But it cannot be that he remains for a long time without mortal sin. Hence Gregory says (Super Ezech. Hom. xi) that " a sin not at once taken away by repentance, by its weight drags us down to other sins": and this because, as the lower appetite ought to be subject to the reason, so should the reason be subject to God, and should place in Him the end of its will. Now it is by the end that all human acts ought to be regulated, even as it is by the judgment of the reason that the movements of the lower appetite should be regulated. And thus, even as inordinate movements of the sensitive appetite cannot help occurring since the lower appetite is not subject to reason, so likewise, since man's reason is not entirely subject to God, the consequence is that many disorders occur in the reason. For when man's heart is not so fixed on God as to be unwilling to be parted from Him for the sake of finding any good or avoiding any evil, many things happen for the achieving or avoiding of which a man strays from God and breaks His commandments, and thus sins mortally: especially since, when surprised, a man acts according to his preconceived end and his pre-existing habits, as the Philosopher says (Ethic. iii); although with premeditation of his reason a man may do something outside the order of his preconceived end and the inclination of his habit. But because a man cannot always have this premeditation, it cannot help occurring that he acts in accordance with his will turned aside from God, unless, by grace, he is quickly brought back to the due order.
Reply to Objection 1. Man can avoid each but not every act of sin, except by grace, as stated above. Nevertheless, since it is by his own shortcoming that he does not prepare himself to have grace, the fact that he cannot avoid sin without grace does not excuse him from sin.
Reply to Objection 2. Correction is useful "in order that out of the sorrow of correction may spring the wish to be regenerate; if indeed he who is corrected is a son of promise, in such sort that whilst the noise of correction is outwardly resounding and punishing, God by hidden inspirations is inwardly causing to will," as Augustine says (De Corr. et Gratia vi). Correction is therefore necessary, from the fact that man's will is required in order to abstain from sin; yet it is not sufficient without God's help. Hence it is written (Ecclesiastes 7:14): "Consider the works of God that no man can correct whom He hath despised."
Reply to Objection 3. As Augustine says (Hypognosticon iii [Among the spurious works of St. Augustine]), this saying is to be understood of man in the state of perfect nature, when as yet he was not a slave of sin. Hence he was able to sin and not to sin. Now, too, whatever a man wills, is given to him; but his willing good, he has by God's assistance.
Source: Summa Theologica (New Advent)