Objection 1. It would seem that man can merit everlasting life without grace. For Our Lord says (Matthew 19:17): "If thou wilt enter into life, keep the commandments"; from which it would seem that to enter into everlasting life rests with man's will. But what rests with our will, we can do of ourselves. Hence it seems that man can merit everlasting life of himself.
Objection 2. Further, eternal life is the wage of reward bestowed by God on men, according to Matthew 5:12: "Your reward is very great in heaven." But wage or reward is meted by God to everyone according to his works, according to Psalm 61:12: "Thou wilt render to every man according to his works." Hence, since man is master of his works, it seems that it is within his power to reach everlasting life.
Objection 3. Further, 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.
On the contrary, The Apostle says (Romans 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 is an end exceeding the proportion of human nature, as is clear from what we have said above (Question 5, Article 5). 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 [Hypognosticon iii, among the spurious works of St. Augustine].
Reply to Objection 1. Man, by his will, does works meritorious of everlasting life; but as Augustine says, in the same book, for this it is necessary that the will of man should be prepared with grace by God.
Reply to Objection 2. As the gloss upon Romans 6:23, "The grace of God is life everlasting," says, "It is certain that everlasting life is meter to good works; but the works to which it is meted, belong to God's grace." And it has been said (4), that to fulfil the commandments of the Law, in their due way, whereby their fulfilment may be meritorious, requires 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 nowise reach; even as a man who can recover his health by the help of medicines is better disposed to health than one who can nowise recover it, as the Philosopher observes (De Coelo ii, 12).
Source: Summa Theologica (New Advent)