Objection 1. It would seem that the actions of the first man were less meritorious than ours are. For grace is given to us through the mercy of God, Who succors most those who are most in need. Now we are more in need of grace than was man in the state of innocence. Therefore grace is more copiously poured out upon us; and since grace is the source of merit, our actions are more meritorious.
Objection 2. Further, struggle and difficulty are required for merit; for it is written (2 Timothy 2:5): "He... is not crowned except he strive lawfully" and the Philosopher says (Ethic. ii, 3): "The object of virtue is the difficult and the good." But there is more strife and difficulty now. Therefore there is greater efficacy for merit.
Objection 3. Further, the Master says (Sent. ii., D, xxiv) that "man would not have merited in resisting temptation; whereas he does merit now, when he resists." Therefore our actions are more meritorious than in the primitive state.
On the contrary, if such were the case, man would be better off after sinning.
I answer that, Merit as regards degree may be gauged in two ways.
First, in its root, which is grace and charity. Merit thus measured corresponds in degree to the essential reward, which consists in the enjoyment of God; for the greater the charity whence our actions proceed, the more perfectly shall we enjoy God.
Secondly, the degree of merit is measured by the degree of the action itself. This degree is of two kinds, absolute and proportional. The widow who put two mites into the treasury performed a deed of absolutely less degree than the others who put great sums therein. But in proportionate degree the widow gave more, as Our Lord said; because she gave more in proportion to her means. In each of these cases the degree of merit corresponds to the accidental reward, which consists in rejoicing for created good.
We conclude therefore that in the state of innocence man's works were more meritorious than after sin was committed, if we consider the degree of merit on the part of grace, which would have been more copious as meeting with no obstacle in human nature: and in like manner, if we consider the absolute degree of the work done; because, as man would have had greater virtue, he would have performed greater works. But if we consider the proportionate degree, a greater reason for merit exists after sin, on account of man's weakness; because a small deed is more beyond the capacity of one who works with difficulty than a great deed is beyond one who performs it easily.
Reply to Objection 1. After sin man requires grace for more things than before sin; but he does not need grace more; forasmuch as man even before sin required grace to obtain eternal life, which is the chief reason for the need of grace. But after sin man required grace also for the remission of sin, and for the support of his weakness.
Reply to Objection 2. Difficulty and struggle belong to the degree of merit according to the proportionate degree of the work done, as above explained. It is also a sign of the will's promptitude striving after what is difficult to itself: and the promptitude of the will is caused by the intensity of charity. Yet it may happen that a person performs an easy deed with as prompt a will as another performs an arduous deed; because he is ready to do even what may be difficult to him. But the actual difficulty, by its penal character, enables the deed to satisfy for sin.
Reply to Objection 3. The first man would not have gained merit in resisting temptation, according to the opinion of those who say that he did not possess grace; even as now there is no merit to those who have not grace. But in this point there is a difference, inasmuch as in the primitive state there was no interior impulse to evil, as in our present state. Hence man was more able then than now to resist temptation even without grace.
Source: Summa Theologica (New Advent)