Objection 1. It would seem that without grace anyone can merit eternal life. For man merits from God what he is divinely ordained to, as stated above (Article 1). Now man by his nature is ordained to beatitude as his end; hence, too, he naturally wishes to be blessed. Hence man by his natural endowments and without grace can merit beatitude which is eternal life.
Objection 2. Further, the less a work is due, the more meritorious it is. Now, less due is that work which is done by one who has received fewer benefits. Hence, since he who has only natural endowments has received fewer gifts from God, than he who has gratuitous gifts as well as nature, it would seem that his works are more meritorious with God. And thus if he who has grace can merit eternal life to some extent, much more may he who has no grace.
Objection 3. Further, God's mercy and liberality infinitely surpass human mercy and liberality. Now a man may merit from another, even though he has not hitherto had his grace. Much more, therefore, would it seem that a man without grace may merit eternal life.
On the contrary, The Apostle says (Romans 6:23): "The grace of God, life everlasting."
I answer that, 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."
Reply to Objection 1. God ordained human nature to attain the end of eternal life, not by its own strength, but by the help of grace; and in this way its act can be meritorious of eternal life.
Reply to Objection 2. Without grace a man cannot have a work equal to a work proceeding from grace, since the more perfect the principle, the more perfect the action. But the objection would hold good, if we supposed the operations equal in both cases.
Reply to Objection 3. With regard to the first reason adduced, the case is different in God and in man. For a man receives all his power of well-doing from God, and not from man. Hence a man can merit nothing from God except by His gift, which the Apostle expresses aptly saying (Romans 11:35): "Who hath first given to Him, and recompense shall be made to him?" But man may merit from man, before he has received anything from him, by what he has received from God.
But as regards the second proof taken from the impediment of sin, the case is similar with man and God, since one man cannot merit from another whom he has offended, unless he makes satisfaction to him and is reconciled.
Source: Summa Theologica (New Advent)