Objection 1. It would seem that the souls of the departed are not assisted only by the prayers of the Church, the sacrifice of the altar and alms, or that they are not assisted by them chiefly. For punishment should compensate for punishment. Now fasting is more penal than almsgiving or prayer. Therefore fasting profits more as suffrage than any of the above.
Objection 2. Further, Gregory reckons fasting together with these three, as stated in the Decretals (xiii, Q. ii, Cap. 22): "The souls of the departed are released in four ways, either by the offerings of priests, or the alms of their friends, or the prayers of the saints, or the fasting of their kinsfolk." Therefore the three mentioned above are insufficiently reckoned by Augustine (De Cura pro Mort. xviii).
Objection 3. Further, Baptism is the greatest of the sacraments, especially as regards its effect. Therefore Baptism and other sacraments ought to be offered for the departed equally with or more than the Sacrament of the altar.
Objection 4. Further, this would seem to follow from the words of 1 Corinthians 15:29, "If the dead rise not again at all, why are they then baptized for them?" Therefore Baptism avails as suffrage for the dead.
Objection 5. Further, in different Masses there is the same Sacrifice of the altar. If, therefore, sacrifice, and not the Mass, be reckoned among the suffrages, it would seem that the effect would be the same whatever Mass be said for a deceased person, whether in honor of the Blessed Virgin or of the Holy Ghost, or any other. Yet this seems contrary to the ordinance of the Church which has appointed a special Mass for the dead.
Objection 6. Further, the Damascene (Serm.: De his qui in fide dormierunt) teaches that candles and oil should be offered for the dead. Therefore not only the offering of the sacrifice of the altar, but also other offerings should be reckoned among suffrages for the dead.
I answer that, The suffrages of the living profit the dead in so far as the latter are united to the living in charity, and in so far as the intention of the living is directed to the dead. Consequently those whose works are by nature best adapted to assist the dead, which pertain chiefly to the communication of charity, or to the directing of one's intention to another person. Now the sacrament of the Eucharist belongs chiefly to charity, since it is the sacrament of ecclesiastical unity, inasmuch as it contains Him in Whom the whole Church is united and incorporated, namely Christ: wherefore the Eucharist is as it were the origin and bond of charity. Again, chief among the effects of charity is the work of almsgiving: wherefore on the part of charity these two, namely the sacrifice of the Church and almsgiving are the chief suffrages for the dead. But on the part of the intention directed to the dead the chief suffrage is prayer, because prayer by its very nature implies relation not only to the person who prays, even as other works do, but more directly still to that which we pray for. Hence these three are reckoned the principal means of succoring the dead, although we must allow that any other goods whatsoever that are done out of charity for the dead are profitable to them.
Reply to Objection 1. When one person satisfies for another, the point to consider, in order that the effect of his satisfaction reach the other, is the thing whereby the satisfaction of one passes to another, rather than even the punishment undergone by way of satisfaction; although the punishment expiates more the guilt of the one who satisfies, in so far as it is a kind of medicine. And consequently the three aforesaid are more profitable to the departed than fasting.
Reply to Objection 2. It is true that fasting can profit the departed by reason of charity, and on account of the intention being directed to the departed. Nevertheless, fasting does not by its nature contain anything pertaining to charity or to the directing of the intention, and these things are extrinsic thereto as it were, and for this reason Augustine did not reckon, while Gregory did reckon, fasting among the suffrages for the dead.
Reply to Objection 3. Baptism is a spiritual regeneration, wherefore just as by generation being does not accrue save to the object generated, so Baptism produces its effect only in the person baptized, as regards the deed done: and yet as regards the deed of the doer whether of the baptizer or of the baptized, it may profit others even as other meritorious works. On the other hand, the Eucharist is the sign of ecclesiastical unity, wherefore by reason of the deed done its effect can pass to another, which is not the case with the other sacraments.
Reply to Objection 4. According to a gloss this passage may be expounded in two ways. First, thus: "If the dead rise not again, nor did Christ rise again, why are they baptized for them? i.e. for sins, since they are not pardoned if Christ rose not again, because in Baptism not only Christ's passion but also His resurrection operates, for the latter is in a sense the cause of our spiritual resurrection." Secondly, thus: There have been some misguided persons who were baptized for those who had departed this life without baptism, thinking that this would profit them: and according to this explanation the Apostle is speaking, in the above words, merely according to the opinion of certain persons.
Reply to Objection 5. In the office of the Mass there is not only a sacrifice but also prayers. Hence the suffrage of the Mass contains two of the things mentioned by Augustine (De Cura pro Mort. xviii), namely "prayer" and "sacrifice." As regards the sacrifice offered the Mass profits equally the departed, no matter in whose honor it be said: and this is the principal thing done in the Mass. But as regards the prayers, that Mass is most profitable in which the prayers are appointed for this purpose. Nevertheless, this defect may be supplied by the greater devotion, either of the one who says Mass, or of the one who orders the Mass to be said, or again, by the intercession of the saint whose suffrage is besought in the Mass.
Reply to Objection 6. This offering of candles or oil may profit the departed in so far as they are a kind of alms: for they are given for the worship of the Church or for the use of the faithful.
Source: Summa Theologica (New Advent)