Objection 1. It would seem that works done without charity merit some, at least a temporal, good. For as punishment is to the evil act, so is reward to a good act. Now no evil deed is unpunished by God the just judge. Therefore no good deed is unrewarded, and so every good deed merits some good.
Objection 2. Further, reward is not given except for merit. Now some reward is given for works done without charity, wherefore it is written (Matthew 6:2-16) of those who do good actions for the sake of human glory, that "they have received their reward." Therefore those works merit some good.
Objection 3. Further, if there be two men both in sin, one of whom does many deeds that are good in themselves and in their circumstances, while the other does none, they are not equally near to the reception of good things from Gods else the latter need not be advised to do any good deeds. Now he that is nearer to God receives more of His good things. Therefore the former, on account of his good works, merits some good from God.
On the contrary, Augustine says that "the sinner is not worthy of the bread he eats." Therefore he cannot merit anything from God.
Further, he that is nothing, can merit nothing. But a sinner, through not having charity, is nothing in respect of spiritual being, according to 1 Corinthians 13:2. Therefore he can merit nothing.
I answer that, Properly speaking a merit is an action on account of which it is just that the agent should be given something. Now justice is twofold: first, there is justice properly so called, which regards something due on the part of the recipient. Secondly, there is metaphorical justice, so to speak, which regards something due on the part of the giver, for it may be right for the giver to give something to which the receiver has no claim. In this sense the "fitness of the Divine goodness" is justice; thus Anselm says (Proslog. x) that "God is just when He spares the sinner, because this is befitting." And in this way merit is also twofold. The first is an act in respect of which the agent himself has a claim to receive something, and this is called merit of "condignity." The second is an act the result of which is that there is a duty of giving in the giver by reason of fittingness, wherefore it is called merit of "congruity." Now since in all gratuitous givings, the primary reason of the giving is love, it is impossible for anyone, properly speaking, to lay claim to a gift, if he lack friendship. Wherefore, as all things, whether temporal or eternal, are bestowed on us by the bounty of God, no one can acquire a claim to any of them, save through charity towards God: so that works done without charity are not condignly meritorious of any good from God either eternal or temporal. But since it is befitting the goodness of God, that wherever He finds a disposition He should grant the perfection, a man is said to merit congruously some good by means of good works done without charity. Accordingly suchlike works avail for a threefold good, acquisition of temporal goods, disposition to grace, habituation to good works. Since, however, this is not merit properly so called, we should grant that such works are not meritorious of any good, rather than that they are.
Reply to Objection 1. As the Philosopher states (Ethic. viii, 14), since no matter what a son may do, he can never give back to his father the equal of what he has received from him a father can never become his son's debtor: and much less can man make God his debtor on account of equivalence of work. Consequently no work of ours can merit a reward by reason of its measure of goodness, but it can by reason of charity, which makes friends hold their possessions in common. Therefore, no matter how good a work may be, if it be done without charity, it does not give man a claim to receive anything from God. On the other hand, an evil deed deserves an equivalent punishment according to the measure of its malice, because no evil has been done to us on the part of God, like the good which He has done. Therefore, although an evil deed deserves condign punishment, nevertheless a good deed without charity does not merit condign reward.
Reply to Objections 2 and 3. These arguments consider merit of congruity; while the other arguments consider merit of condignity.
Source: Summa Theologica (New Advent)