There is a very long article
here by Jimmy Akin about Righteousness and Merit and the differences in the Protestant and Catholic uses of the words. It is in three sections.
I. Words And Word-Fights
II. Righteousnessritumk
III. Merit
I'll try and summarise the section on Merit by quoting some key points from it..
As I understand it, most Protestants equate merit with earn. So works that merit are earning something.
In Catholic theology merit equates with
reward so that works that merit are
rewarded. So rather than meritorious acts we should think of them as
rewardable acts. Meritum is the Latin translation of reward.
Akin says:
Brought about by God's grace, acts which please God are done by Christians (Phil. 4:18, Col. 1:9-10, 1Th. 4:1, Heb. 13:16, 13:20-21) and God chooses to reward them (Rom. 2:6, 1 Cor. 3:8, 4:6, 2 Cor. 5:10, Gal. 6:6-10, Rev. 2:23, 22:12). These elements, God's grace, the acts pleasing to God that they bring about, and the reward God chooses to give, are the key elements in the Catholic theology of merit, as we shall see.
There are three forms of merit - congruous, condign, and strict.
In all three forms, there is a similarity between the action and the reward, and it is this similarity which makes it fitting for the reward to be given for that work, which is why the term "merit" is applied. In all cases of merit, an action merits its reward in the sense that the action is similar to the reward in a certain way and thus makes it fitting that the reward be given. The difference between the kinds of merit depends on the kind of similarity between the action and the reward and, correspondingly, it depends on the kind of fittingness there is that the action be given the reward.
The basic principle of supernatural merit, …the thing that makes it supernatural, is the grace which God gives to enable there to be a supernatural act in the first place, the only kind of act for which a supernatural reward is fitting
Congruent Merit
Congruent merit occurs with respect to God when a person under the influence of actual grace does an action which pleases God but which he has not promised to reward. Some times God chooses to reward the act, sometimes not. For example, if we obey Jesus' instruction to supernaturally love our enemies and pray for them; however, God has not promised that he will answer our prayers concerning them, and although he is pleased with the prayers we are offering out of supernatural love for them, he may not give them the blessing we are asking for them. It may simply not be God's will for that to happen. The same is true of prayers for ourselves; even when we pray from supernatural charity we are likely only congruently meriting the thing we are asking for since God has not promised to give it.
Condign Merit
The obvious next higher form of merit is one in which God has promised to reward the action. In this case when a person under the influence of actual graces performs the supernatural act, God is not only pleased by the act but he is guaranteed to reward it because he has promised to do so. This kind of merit is known in Catholic theology as condign merit.
An important point in both cases is that the reward God gives is not equal in value to the act which God rewards, but superior to it because it is supernatural.
Strict Merit
If our actions were equal in value to his reward then what would have occurred would be referred to in modern Catholic parlance as strict merit. Strict merit is what would occur when someone gives to God something of equal intrinsic value to the reward he has promised to give. The trick is, only Christ is capable of doing this since only Christ is capable of doing things of infinite value for God. Other humans are totally incapable of this because we lack the infinite dignity of the Godhead supervening on our actions.
