Objection 1. It would seem that a man can merit the first grace for another. Because on Matthew 9:2: "Jesus seeing their faith," etc. a gloss says: "How much is our personal faith worth with God, Who set such a price on another's faith, as to heal the man both inwardly and outwardly!" Now inward healing is brought about by grace. Hence a man can merit the first grace for another.
Objection 2. Further, the prayers of the just are not void, but efficacious, according to James 5:16: "The continued prayer of a just man availeth much." Now he had previously said: "Pray one for another, that you may be saved." Hence, since man's salvation can only be brought about by grace, it seems that one man may merit for another his first grace.
Objection 3. Further, it is written (Luke 16:9): "Make unto you friends of the mammon of iniquity, that when you shall fail they may receive you into everlasting dwellings." Now it is through grace alone that anyone is received into everlasting dwellings, for by it alone does anyone merit everlasting life as stated above (2; 109, 5). Hence one man may by merit obtain for another his first grace.
On the contrary, It is written (Jeremiah 15:1): "If Moses and Samuel shall stand before Me, My soul is not towards this people" --yet they had great merit with God. Hence it seems that no one can merit the first grace for another.
I answer that, As shown above (1,3,4), our works are meritorious from two causes: first, by virtue of the Divine motion; and thus we merit condignly; secondly, according as they proceed from free-will in so far as we do them willingly, and thus they have congruous merit, since it is congruous that when a man makes good use of his power God should by His super-excellent power work still higher things. And therefore it is clear that no one can merit condignly for another his first grace, save Christ alone; since each one of us is moved by God to reach life everlasting through the gift of grace; hence condign merit does not reach beyond this motion. But Christ's soul is moved by God through grace, not only so as to reach the glory of life everlasting, but so as to lead others to it, inasmuch as He is the Head of the Church, and the Author of human salvation, according to Hebrews 2:10: "Who hath brought many children into glory [to perfect] the Author of their salvation."
But one may merit the first grace for another congruously; because a man in grace fulfils God's will, and it is congruous and in harmony with friendship that God should fulfil man's desire for the salvation of another, although sometimes there may be an impediment on the part of him whose salvation the just man desires. And it is in this sense that the passage from Jeremias speaks.
Reply to Objection 1. A man's faith avails for another's salvation by congruous and not by condign merit.
Reply to Objection 2. The impetration of prayer rests on mercy, whereas condign merit rests on justice; hence a man may impetrate many things from the Divine mercy in prayer, which he does not merit in justice, according to Daniel 9:18: "For it is not for our justifications that we present our prayers before Thy face, but for the multitude of Thy tender mercies."
Reply to Objection 3. The poor who receive alms are said to receive others into everlasting dwellings, either by impetrating their forgiveness in prayer, or by meriting congruously by other good works, or materially speaking, inasmuch as by these good works of mercy, exercised towards the poor, we merit to be received into everlasting dwellings.
Source: Summa Theologica (New Advent)