St. Thomas on Justification - Salvation by works is impossible

We cannot win our way to heaven on the basis of our own works, because it is beyond the power of human nature to do so.

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 Corinthians 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. [ST I-II, Q5, A5; note that here St. Thomas uses Aristotelian language with respect to man's final end; that Happiness which we seek is heaven, the Beatific Vision of God]

It's worth pointing out, too, that Aquinas is not even considering the problem of sin, as we shall see:

[M]an's perfect Happiness, as stated above (Question 3, Article 8), 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 I, 12, 4. For the natural knowledge of every creature is in keeping with the mode of his substance: thus it is said of the intelligence (De Causis; Prop. viii) 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. [I-II, Q5, A5]

Even if we were to disregard the fact that our sins prevent us from seeing God, by our very nature we are not capable of attaining to the Beatific Vision.

It is written: "The grace of God is life everlasting" (Romans 6:23). But life everlasting consists in the vision of the Divine essence, according to the words: "This is eternal life, that they may know Thee the only true God," etc. (John 17:3). Therefore to see the essence of God is possible to the created intellect by grace, and not by nature. [I, Q12, A4]

If life everlasting is knowledge of God in the Beatific Vision, and if the grace of God is life everlasting - as the two passages St. Thomas quotes here say - then we can only see God by grace.

Consequently the problem for those who say that Catholics believe a "works-based" salvation is twofold. As we have seen in this series, we cannot be justified by anything that we do; Christ alone forgives our sins. And even when our sins are forgiven, it is impossible for us to attain to eternal life on the basis of our own efforts, because an eternal goal is beyond the measure of our finite powers. It is only by God's grace that we can fulfill our final End; it is only by his grace that we can spend Eternity with him. Those who say otherwise about us are badly misinformed.