Objection 1. It would seem that the suffrages of one person cannot profit others. For it is written (Galatians 6:8): "What things a man shall sow, those also shall he reap." Now if one person reaped fruit from the suffrages of another, he would reap from another's sowing. Therefore a person receives no fruit from the suffrages of others.
Objection 2. Further, it belongs to God's justice, that each one should receive according to his merits, wherefore the psalm (Psalm 61:13) says: "Thou wilt render to every man according to his works." Now it is impossible for God's justice to fail. Therefore it is impossible for one man to be assisted by the works of another.
Objection 3. Further, a work is meritorious on the same count as it is praiseworthy, namely inasmuch as it is voluntary. Now one man is not praised for the work of another. Therefore neither can the work of one man be meritorious and fruitful for another.
Objection 4. Further, it belongs to Divine justice to repay good for good in the same way as evil for evil. But no man is punished for the evildoings of another; indeed, according to Ezekiel 18:4, "the soul that sinneth, the same shall die." Therefore neither does one person profit by another's good.
On the contrary, It is written (Psalm 118:63): "I am a partaker with all them that fear Thee," etc.
Further, all the faithful united together by charity are members of the one body of the Church. Now one member is assisted by another. Therefore one man can be assisted by the merits of another.
I answer that, our actions can avail for two purposes. First, for acquiring a certain state; thus by a meritorious work a man obtains the state of bliss. Secondly, for something consequent upon a state; thus by some work a man merits an accidental reward, or a rebate of punishment. And for both these purposes our actions may avail in two ways: first, by way of merit; secondly, by way of prayer: the difference being that merit relies on justice, and prayer on mercy; since he who prays obtains his petition from the mere liberality of the one he prays. Accordingly we must say that the work of one person nowise can avail another for acquiring a state by way of merit, so that, to wit, a man be able to merit eternal life by the works which I do, because the share of glory is awarded according to the measure of the recipient, and each one is disposed by his own and not by another's actions--disposed, that is to say, by being worthy of reward. By way of prayer, however, the work of one may profit another while he is a wayfarer, even for acquiring a state; for instance, one man may obtain the first grace for another [Cf. I-II, 114, 6]: and since the impetration of prayer depends on the liberality of God Whom we pray, it may extend to whatever is ordinately subject to the Divine power. On the other hand, as regards that which is consequent upon or accessory to a state, the work of one may avail another, not only by way of prayer but even by way of merit: and this happens in two ways. First, on account of their communion in the root of the work, which root is charity in meritorious works. Wherefore all who are united together by charity acquire some benefit from one another's works, albeit according to the measure of each one's state, since even in heaven each one will rejoice in the goods of others. Hence it is that the communion of saints is laid down as an article of faith. Secondly, through the intention of the doer who does certain works specially for the purpose that they may profit such persons: so that those works become somewhat the works of those for whom they are done, as though they were bestowed on them by the doer. Wherefore they can avail them either for the fulfillment of satisfaction or for some similar purpose that does not change their state.
Reply to Objection 1. This reaping is the receiving of eternal life, as stated in John 4:36, "And he that reapeth... gathereth fruit unto life everlasting." Now a share of eternal life is not given to a man save for his own works, for although we may impetrate for another that he obtain life, this never happens except by means of his own works, when namely, at the prayers of one, another is given the grace whereby he merits eternal life.
Reply to Objection 2. The work that is done for another becomes his for whom it is done: and in like manner the work done by a man who is one with me is somewhat mine. Hence it is not contrary to Divine justice if a man receives the fruit of the works done by a man who is one with him in charity, or of works done for him. This also happens according to human justice, so that the satisfaction offered by one is accepted in lieu of another's.
Reply to Objection 3. Praise is not given to a person save according to his relation to an act, wherefore praise is "in relation to something" (Ethic. i, 12). And since no man is made or shown to be well- or ill-disposed to something by another's deed, it follows that no man is praised for another's deeds save accidentally in so far as he is somewhat the cause of those deeds, by giving counsel, assistance, inducement, or by any other means. on the other hand, a work is meritorious to a person, not only by reason of his disposition, but also in view of something consequent upon his disposition or state, as evidenced by what has been said.
Reply to Objection 4. It is directly contrary to justice to take away from a person that which is his due: but to give a person what is not his due is not contrary to justice, but surpasses the bounds of justice, for it is liberality. Now a person cannot be hurt by the ills of another, unless he be deprived of something of his own. Consequently it is not becoming that one should be punished for another's sins, as it is that one should acquire some advantage from deeds of another.
Source: Summa Theologica (New Advent)