In the following articles, from the Summa Theologica, St Thomas refutes various points, which apply perfectly to the teaching of Buddhism. Can one attain perfection, that is observe all the commandments, avoid sin, reach Heaven, without the help of God?  The Buddhists answer yes, since they do not recognize the necessity of the grace of Our Lord Jesus Christ.  St Thomas, the Common Doctor of the Church says, with all the Fathers, with all the Tradition and teaching of the Church, no, absolutely no.

Augustine says that it is part of the Pelagian heresy that ?they believe that without grace man can fulfil all the Divine commandments.?

I answer that, There are two ways of fulfilling the commandments of the Law. The first regards the substance of the works, as when a man does works of justice, fortitude, and of other virtues. And in this way man in the state of perfect nature (i.e. before the fall - Ed.) could fulfil all the commandments of the Law; otherwise he would have been unable to sin in that state, since to sin is nothing else than to transgress the Divine commandments. But in the state of corrupted nature man cannot fulfil all the Divine commandments without healing grace. Secondly, the commandments of the law can be fulfilled, not merely as regards the substance of the act, but also as regards the mode of acting, i.e. their being done out of charity. And in this way, neither in the state of perfect nature, nor in the state of corrupt nature can man fulfil the commandments of the law without grace. Hence, Augustine having stated that ?without grace men can do no good whatever,? adds: ?Not only do they know by its light what to do, but by its help they do lovingly what they know.? Beyond this, in both states they need the help of God's motion in order to fulfil the commandments.

The Apostle says (Rm. 6:23): ?The grace of God is life everlasting.? And as a gloss says, this is said ?that we may understand that God, of His own mercy, leads us to everlasting life.?

I answer that, Acts conducing to an end must be proportioned to the end. But no act exceeds the proportion of its active principle; and hence we see in natural things, that nothing can by its operation bring about an effect which exceeds its active force, but only such as is proportionate to its power. Now everlasting life iman end exceeding the proportion of human nature, as is clear from what we have said above (1a2ae q5, a5, see next article below). Hence man, by his natural endowments, cannot produce meritorious works proportionate to everlasting life; and for this a higher force is needed, viz. the force of grace. And thus without grace man cannot merit everlasting life; yet he can perform works conducing to a good which is natural to man, as ?to toil in the fields, to drink, to eat, or to have friends,? and the like, as Augustine says in his third Reply to the Pelagians 

Objection 3. Everlasting life is the last end of human life. Now every natural thing by its natural endowments can attain its end. Much more, therefore, may man attain to life everlasting by his natural endowments, without grace.

Reply to Objection 3. This objection has to do with the natural end of man. Now human nature, since it is nobler, can be raised by the help of grace to a higher end, which lower natures can not reach; even as a man who can recover his health by the help of medicines is better disposed to health than one who can not recover it, as the philosopher observes.

Man is naturally the principle of his action, by his intellect and will. But final Happiness prepared for the saints, surpasses the intellect and will of man; for the Apostle says (1 Cor. 2:9) "Eye hath not seen, nor ear heard, neither hath it entered into the heart of man, what things God hath prepared for them that love Him." Therefore man cannot attain Happiness by his natural powers.

I answer that, Imperfect happiness that can be had in this life, can be acquired by man by his natural powers, in the same way as virtue, in whose operation it consists: on this point we shall speak further on (q63). But man's perfect Happiness, as stated above (q3, a8), consists in the vision of the Divine Essence. Now the vision of God's Essence surpasses the nature not only of man, but also of every creature, as was shown in the 1a, q12, a4. For the natural knowledge of every creature is in keeping with the mode of his substance: thus it is said of the intelligence that "it knows things that are above it, and things that are below it, according to the mode of its substance." But every knowledge that is according to the mode of created substance, falls short of the vision of the Divine Essence, which infinitely surpasses all created substance. Consequently neither man, nor any creature, can attain final Happiness by his natural powers.

Objection 1. It would seem that man can attain happiness by his natural powers. For nature does not fail in necessary things. But nothing is so necessary to man as that by which he attains the last end. Therefore this is not lacking to human nature. Therefore man can attain Happiness by his natural powers.

Reply to Objection 1. Just as nature does not fail man in necessaries, although it has not provided him with weapons and clothing, as it provided other animals, because it gave him reason and hands, with which he is able to get these things for himself; so neither did it fail man in things necessary, although it gave him not the wherewithal to attain Happiness: since this it could not do. But it did give him free-will, with which he can turn to God, that He may make him happy. "For what we do by means of our friends, is done, in a sense, by ourselves" (Ethic. III, 3).

Augustine says: "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 (Rm. 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 (q74, a5); 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 (q74, a3, 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 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 rodu 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; 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.

Objection 1. It would seem that without grace man can avoid sin. Because "no one sins in what he cannot avoid," as Augustine says. Hence if a man in mortal sin cannot avoid sin, it would seem that in sinning he does not sin, which is impossible.

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 grn en the fact that he cannot avoid sin without grace does not excuse him from sin.