Buying and selling belong to the so-called contracts of exchange, buyer and seller exchange definite objects, and as in accordance with the virtue of justice we are each obliged to give what is ours in proportion to our rights, the objects to be exchanged must represent an approximately equal value, and therefore, in the nature of things all kinds of factors are drawn into the calculation. The same happens with merits and reward. Here too there is a contract of exchange in question, but with this difference that the one who merits offers no objects for exchange, but offers his activity in exchange for the reward which may or may not consist in an object.
But justice is a virtue that regulates the attitude of men to one another, and therefore in the strict sense of the word we can be just only with regard to another. If there is anything lacking in this respect, so that there is no question of another in the full meaning of the word, that will entail special consequences.
Such a case is that of a child who is still young. It is not yet independent: it still belongs to its father, and therefore neither father nor child can count as separate from one another. So that if a child under age is on his father’s payroll, not only the youth himself, but his work belongs to his father. That son cannot claim any wages in virtue of justice, as can other workers in the same employment. He could lay claim to wages only in so far as his father had promised them and is therefore obliged to pay them in virtue of fidelity to his promise.
But far more closely than a child under age belongs to his father, does the creature belong to God. It will never be able to lay claim to any remuneration in the name of justice. The only question of reward would be in as far as God had promised it of his own free will, and hence owes it to himself to make good his promise.
So that if we speak of human merits in connection with God, these can never be the same kind of merits as men have in respect to one another, for man, with all he has and all he does, belongs wholly and unreservedly to God.
The 19th Ecumenical Council of Trent teaches us on this subject that God, for the sake of Jesus’ Passion, gave us grace, by which we can effectively merit: increase of the same grace, eternal life, the winning of that eternal life (on condition of dying in the state of grace), and increase of heavenly glory. (Denzinger 842)
In his omnipotence, God promised us that reward for the works we should accomplish by the strength of grace. They are really merits, but they do not depend on justice but on God’s free promise, and hence it is necessary to make a restriction: they are indeed merits, but they are not that alone and nothing else, and consequently we call them merits in a limited respect, namely in consideration of God’s free promise.
There is a great difference to be found between our merits and those of our Lord and Savior Jesus Christ.
The first difference concerns this very point. The relations of the three divine Persons are in the fullest sense those of “another to another.”
What Jesus merits is therefore not only real merit, but also real merit simply and unreservedly; thus the eternal Father is bound not only to himself but also to Jesus to give him the reward agreed upon, and that without a preliminary free promise.