On the contrary, Augustine says (De Natura et Gratia xxvi) that "as the eye of the body though most healthy cannot see unless it is helped by the brightness of light, so, neither can a man, even if he is most righteous, live righteously unless he be helped by the eternal light of justice." But justification is by grace, according to Romans 3:24: "Being justified freely by His grace." Hence even a man who already possesses grace needs a further assistance of grace in order to live righteously. [ST I-II, Q109, A9; emphasis added]From this it appears that it is not possible for a man to save himself or earn anything from God on his own. Aquinas continues:
As stated above (Article 5 [and as we saw here - RdP]), in order to live righteously a man needs a twofold help of God--first, a habitual gift whereby corrupted human nature is healed, and after being healed is lifted up so as to work deeds meritoriously of everlasting life, which exceed the capability of nature. Secondly, man needs the help of grace in order to be moved by God to act.We have no hope of heaven apart from grace. This does not mean that we do nothing, though. We must still strive after holiness. But these works of holiness are entirely dependent upon the grace of God, and apart from them we could not do them; and their entire merit is from the grace of God, so that there is no way in which we may say of ourselves that we have merited anything in and of ourselves.
Now with regard to the first kind of help, man does not need a further help of grace, e.g. a further infused habit. Yet he needs the help of grace in another way, i.e. in order to be moved by God to act righteously, and this for two reasons: first, for the general reason that no created thing can put forth any act, unless by virtue of the Divine motion. Secondly, for this special reason--the condition of the state of human nature. For although healed by grace as to the mind, yet it remains corrupted and poisoned in the flesh, whereby it serves "the law of sin," Romans 7:25. In the intellect, too, there seems the darkness of ignorance, whereby, as is written (Romans 8:26): "We know not what we should pray for as we ought"; since on account of the various turns of circumstances, and because we do not know ourselves perfectly, we cannot fully know what is for our good, according to Wisdom 9:14: "For the thoughts of mortal men are fearful and our counsels uncertain." Hence we must be guided and guarded by God, Who knows and can do all things. For which reason also it is becoming in those who have been born again as sons of God, to say: "Lead us not into temptation," and "Thy Will be done on earth as it is in heaven," and whatever else is contained in the Lord's Prayer pertaining to this.
Now there are Protestants who object to saying that we must obey God in order to be saved, even when such obedience is wholly given and founded upon grace as we have seen. This objection is nonsensical when the Protestants in question nevertheless say that we must obey God, and that those who do not obey God cannot be saved. To say the one negatively is but the contrapositive of the other, and both are true.
Now the works of the flesh are manifest: which are fornication, uncleanness, immodesty, luxury, Idolatry, witchcrafts, enmities, contentions, emulations, wraths, quarrels, dissensions, sects, Envies, murders, drunkenness, revellings, and such like. Of the which I foretell you, as I have foretold to you, that they who do such things shall not obtain the kingdom of God. [Gal. 5:19-21]This obviously means that what we Christians do matters, because Paul addresses these remarks to Christians, and warns them against such deeds (verses 16 & 21). How would it make sense to warn the Galatians about this if it could not possibly affect them? It wouldn't.
And if a brother or sister be naked and want daily food: And one of you say to them: Go in peace, be ye warmed and filled; yet give them not those things that are necessary for the body, what shall it profit? So faith also, if it have not works, is dead in itself. [James 2:15-17]And what sense would it make for the writer of Hebrews to warn Christians that they could lose their salvation if they can't lose their salvation?
For it is impossible for those who were once illuminated, have tasted also the heavenly gift and were made partakers of the Holy Ghost, Have moreover tasted the good word of God and the powers of the world to come, And are fallen away: to be renewed again to penance, crucifying again to themselves the Son of God and making him a mockery. [Heb. 6:4-6]Some (who hold that "the elect" cannot lose their salvation) will deny that the Elect are in view here, but really: do they really expect us to believe that on their own view of things non-elect men have been illuminated? That the non-elect have "tasted the heavenly gift," or that they have been "partakers of the Holy Ghost"? These are precisely the things that they say the Elect enjoy! But the author of Hebrews says it is precisely people who have enjoyed these things who nevertheless may fall away, if they are not careful of what they do. But if they must be careful of what they do, then what they do actually has a bearing on their eternal future. And this is of course what Christ says in the parable of the sheep and the goats: what we do matters.
But as St. Thomas says here, and as he says elsewhere too, we cannot obey God so as to attain salvation apart from his grace. The reward we receive is nothing but God rewarding his own gifts. We cannot justify ourselves; we need God's grace. We cannot obey God ourselves; we need God's grace. It is always grace. But grace is simply not so alien to our deeds so as suggest the irrational nonsense that where deeds are present, grace cannot be (as some foolishly suggest).