223. By "merit" we mean a title to reward. It is called "condign" or strict merit, when the reward is due in justice; that is, when the person earning it acquires a definite right against a definite person, who is bound in justice to pay him. Merit is called "congruous" when is gives no right in justice, hut only raises a claim to the generosity of another, which it would be unhandsome in him to disregard. But all merit, condign or congruous, supposes that the action done redounds in some way to the advantage of the person to whom it appeals, and that it is not already due to him.
224. Strictly speaking, no action of man can be of any advantage to God; and therefore Christ tells His disciples: "When you shall have done all these things which are commanded you, say we are unprofitable servants, we have done that which we ought to do" (Luke XVII, 10). But God, in His bounty has deigned, of His own free choice, to promise us a reward for good works, as if they benefited Him; and He owes it to Himself to keep His promises. In this sense, then we have a right to be supernaturally rewarded for supernatural acts: we merit in justice, or condignly. That we do so was defined at Trent (Sess. 6, can. 32); and it is taught by St. Paul, who says: "There is laid up for me a crown of justice, which the Lord, the just judge, will render to me in that day; and not only to me, but to them also that love His coming" (2. Tim. IV, 8).
225. But no supernatural reward is promised, or in any way due, for merely natural acts; for the effect cannot be greater than the cause. Besides, as merit is like the fruit which the soul produces for Heaven, and no fruit can grow upon a dead tree, a soul in grievous sin can gain no merit for Heaven. When we are not in sin, the life of grace flows into us from Christ, as sap from the trunk into the branches of the vine: "I am the vine and you the branches: he that abideth in Me and I in him, the same beareth much fruit; for without Me you can do nothing" (Jo. XV, 5). The intention too must be supernatural, that it may have a proportion to a supernatural reward (n. 206). With the end of life the time for meriting ceases; for "The night cometh when no man can work" (Jo. IX, 4).
The first grace cannot be merited at all (n. 207). With the aid of grace both sinner and just man can congruously merit further actual grace; the just can thus merit final perseverance (n. 208), and he can condignly merit increase of habitual grace, eternal life and increase of glory.
All merit is lost when mortal sin is committed. When grace is recovered, it is the consentient opinion of theologians that the former merit is restored. They infer this from the text "For God is not unjust, that He should forget your work and the love which you have shown in His name" (Hebr. VI, 10). If merit were not restored, the loss would not be wholly repaired, yet the sin is certainly wholly forgiven; which seems to be inconsistent.
In this whole treatise "On Grace" we have quoted the Fathers but rarely, because the Protestant Reformers acknowledged that their own doctrine on this matter was an innovation, and they gloried in the fact; they granted that the Fathers are with us.