Here are some preliminaries:
- No creature can merit anything from God without God himself determining that some created action deserves a reward. This is because God is infinitely greater than any creature and the source of all its goodness, so God never owes man anything in the strictest sense.
- There are two ways of meriting often referred to by St. Thomas and other doctors: de condigno and de congruo.
- To merit de condigno, is to merit worthily, to really in some way deserve the reward that is given.
- To merit de congruo, is to receive a reward due to a kind of fittingness, but not according to justice in the strict sense. For example, a friend may have reason to expect a gift on his birthday (one might even say he deserves it), but this exceeds the realm of justice. The one giving the reward is not bound to do so, but is reasonable in doing so.
- Before grace enters the soul, man cannot merit this grace. The first grace is wholly gratuitous.
- In a righteous action, there are two principles to consider: the human will and grace.
- As far as the human will goes, there can be no meriting de condigno, no meriting according to justice. The human will is a creature infinitely unequal to God.
- Yet the human will can merit de congruo, if by it man does some action for which God has designated a reward.
- With regard to grace as principle, man can merit de condigno. This is because grace is really a participation in divine life. Since God himself deserves all the glory that agrees with his nature, so also we inasmuch as we participate in his divinity deserve all that is becoming to him.
- We know the answer to this question must be yes, at least in the case of Christ.
- Christ, by his death, merits that removal of our sins and the sharing of divine life within us.
- Since Christ is divine, he deserves all things according to justice, and therefore he merits eternal life for us de condigno.
- As for other men, they cannot merit eternal life for others de condigno. God does not owe anything to any man according to strict justice.
- Yet, "You are my friends if you do what I command." Those who possess the grace of God become his friends, and it is fitting that friends answer requests when it is possible to do so. And all things are possible for God.
- So a man merit eternal life for another de congruo, insofar as he is friends with God and asks him this favor.
- Although grace is present in her in a super-eminent way (for she is full of grace and the Almighty has done great things for her), she possesses divine life only by participation.
- Just as with other men, it is also the case that she cannot merit eternal life for other men de condigno. She is a creature, so there is an infinite distance between her and God.
- Yet on account of grace, she can merit for other de congruo, and more so than anyone else on account of her closeness to her own Son.
- So when Mary is called "co-redemptrix", this is meant to refer to her role in meriting eternal life for us de congruo, in a manner exceeding any other creature.
- This may seem misleading, as it seems to indicate an equality with the redemptive action of Christ. Such a position is not tenable.
Places to look at:
http://www.newadvent.org/summa/2114.htm (all of the articles here)
http://www.newadvent.org/summa/4002.htm#article11 (article about merit)
There are more related articles. Just about anything related to justice or grace will be relevant in some way.
Another note: This may seem Thomas-heavy, but he often returns to Scripture and a more thorough understanding of this whole question will require returning to Scripture and seeing what is presented for our benefit.