St. Thomas on Justification - We cannot merit eternal life on our own

Citing Romans 6:23, St. Thomas says that grace is eternal life and consequently is not a thing we can merit on our own.

Man without grace may be looked at in two states, as was said above (Question 109, Article 2): the first, a state of perfect nature, in which Adam was before his sin; the second, a state of corrupt nature, in which we are before being restored by grace. Therefore, if we speak of man in the first state, there is only one reason why man cannot merit eternal life without grace, by his purely natural endowments, viz. because man's merit depends on the Divine pre-ordination. Now no act of anything whatsoever is divinely ordained to anything exceeding the proportion of the powers which are the principles of its act; for it is a law of Divine providence that nothing shall act beyond its powers. Now everlasting life is a good exceeding the proportion of created nature; since it exceeds its knowledge and desire, according to 1 Corinthians 2:9: "Eye hath not seen, nor ear heard, neither hath it entered into the heart of man." And hence it is that no created nature is a sufficient principle of an act meritorious of eternal life, unless there is added a supernatural gift, which we call grace. But if we speak of man as existing in sin, a second reason is added to this, viz. the impediment of sin. For since sin is an offense against God, excluding us from eternal life, as is clear from what has been said above (71, 6; 113, 2), no one existing in a state of mortal sin can merit eternal life unless first he be reconciled to God, through his sin being forgiven, which is brought about by grace. For the sinner deserves not life, but death, according to Romans 6:23: "The wages of sin is death." [ST I-II, Q114, A2; emphasis added]

Hence without grace we cannot be saved; hence our own works cannot save us.