Objection 1. It would seem that suffrages offered for one deceased person are not more profitable to the one for whom they are offered, than to others. For spiritual light is more communicable than a material light. Now a material light, for instance of a candle, though kindled for one person only, avails equally all those who are gathered together, though the candle be not lit for them. Therefore, since suffrages are a kind of spiritual light, though they be offered for one person in particular, do not avail him any more than the others who are in purgatory.
Objection 2. Further, as stated in the text (Sent. iv, D, 45), suffrages avail the dead "in so far as during this life they merited that they might avail them afterwards" [St. Augustine, Enchiridion cx]. Now some merited that suffrages might avail them more than those for whom they are offered. Therefore they profit more by those suffrages, else their merits would be rendered unavailing.
Objection 3. Further, the poor have not so many suffrages given them as the rich. Therefore if the suffrages offered for certain people profit them alone, or profit them more than others, the poor would be worse off: yet this is contrary to our Lord's saying (Luke 6:20): "Blessed are ye poor, for yours is the kingdom of God."
On the contrary, Human justice is copied from Divine justice. But if a person pay another's debt human justice releases the latter alone. Therefore since he who offers suffrages for another pays the debt, in a sense, of the person for whom he offers them, they profit this person alone.
Further, just as a man by offering suffrages satisfies somewhat for a deceased person, so, too, sometimes a person can satisfy for a living person. Now where one satisfies for a living person the satisfaction counts only for the person for whom it is offered. Therefore one also who offers suffrages profits him alone for whom he offers them.
I answer that, There have been two opinions on this question. Some, like Praepositivus, have said that suffrages offered for one particular person do avail chiefly, not the person for whom they are offered, but those who are most worthy. And they instanced a candle which is lit for a rich man and profits those who are with him no less than the rich man himself, and perhaps even more, if they have keener sight. They also gave the instance of a lesson which profits the person to whom it is given no more than others who listen with him, but perhaps profits these others more, if they be more intelligent. And if it were pointed out to them that in this case the Church's ordinance in appointing certain special prayers for certain persons is futile, they said that the Church did this to excite the devotion of the faithful, who are more inclined to offer special than common suffrages, and pray more fervently for their kinsfolk than for strangers.
Others, on the contrary, said that suffrages avail more those for whom they are offered. Now both opinions have a certain amount of truth: for the value of suffrages may be gauged from two sources. For their value is derived in the first place from the virtue of charity, which makes all goods common, and in this respect they avail more the person who is more full of charity, although they are not offered specially for him. In this way the value of suffrages regards more a certain inward consolation by reason of which one who is in charity rejoices in the goods of another after death in respect of the diminution of punishment; for after death there is no possibility of obtaining or increasing grace, whereas during life the works of others avail for this purpose by the virtue of charity. In the second place suffrages derive their value from being applied to another person by one's intention. In this way the satisfaction of one person counts for another, and there can be no doubt that thus they avail more the person for whom they are offered: in fact, they avail him alone in this way, because satisfaction, properly speaking, is directed to the remission of punishment. Consequently, as regards the remission of punishment, suffrages avail chiefly the person for whom they are offered, and accordingly there is more truth in the second opinion than in the first.
Reply to Objection 1. Suffrages avail, after the manner of a light, in so far as they reach the dead, who thereby receive a certain amount of consolation: and this is all the greater according as they are endowed with a greater charity. But in so far as suffrages are a satisfaction applied to another by the intention of the offerer, they do not resemble a light, but rather the payment of a debt: and it does not follow, if one person's debt be paid, that the debt of others is paid likewise.
Reply to Objection 2. Such a merit is conditional, for in this way they merited that suffrages would profit them if offered for them, and this was merely to render themselves fit recipients of those suffrages. It is therefore clear that they did not directly merit the assistance of those suffrages, but made themselves fit by their preceding merits to receive the fruit of suffrages. Hence it does not follow that their merit is rendered unavailing.
Reply to Objection 3. Nothing hinders the rich from being in some respects better off than the poor, for instance as regards the expiation of their punishment. But this is as nothing in comparison with the kingdom of heaven, where the poor are shown to be better off by the authority quoted.
Source: Summa Theologica (New Advent)