A few weeks ago, I posted rather extensively on the reality of Purgatory as a fundamental of Judeo-Christian belief. There are manifold justifications for belief in Purgatory, and only one or two ill-interpreted sentences from St. Paul's writings that could be construed in such a way as to disagree. Where the "rubber meets the road," however, is in interpreting Scripture in light of the belief of the Church, and not the other way around. Christians are called to be a People of the Word, but not a People of the Book. After all, if we get right down to it, in a sort of chicken-and-egg discussion, which came first, the Church or the Bible? The Church! The Church does not profess that which she believes because the Bible says so. The Bible says what it says because it reflects the beliefs of the Church. But I digress, as this is a whole other discussion to be had a later time.
With regard to prayers for the dead, and most especially Masses for the dead, we take our queue from 2 Maccabees, in which Judas Maccabeus sends 12,000 pieces of silver to Jerusalem to the temple that an expiatory sacrifice be offered to free his fallen comrades from their sins. He says that "It is therefore a holy and wholesome thought to pray for the dead, that they may be loosed from sins." (12:43-46). Indeed, Judas' justification for praying for the dead is closely linked to a belief in the resurrection of the dead: For if he had not hoped that they that were slain should rise again, it would have seemed superfluous and vain to pray for the dead.
This practice of praying for the dead, then, pre-exists Christianity, and remains a necessary teaching of the Christian Faith. Our friends over at Fisheaters.com have this to say on the matter:
Those who've died in a state of grace are not truly "dead"; they are our beloved in Heaven or in Purgatory (on their way to Heaven) and will forever be, world without end, part of the Communion of Saints -- the Church Triumphant (the Saints in Heaven, whether or not they are beatified or canonized), the Church Suffering (the saints in Purgatory), and the Church Militant (the saints on earth).
Because we can't know, aside from those the Church has beatified or canonized, who is already in Heaven, who is in Purgatory for a time, or who is damned, we pray for the dead for the rest of our lives -- assuming they are in Purgatory, while hoping they are in Heaven and not damned.
We also ask those who've died to pray for us. While those whom the Church has deemed to be of the Church Triumphant (the canonized Saints) are in Heaven for certain and are, therefore, in no need of our prayers for them, we've always asked for them to pray for us. As to the Church Suffering in Purgatory, Aquinas teaches that they are not able to know, by themselves, our prayers; however, it is piously believed, and taught by St. Alphonsus Liguori, that God makes our prayers known to them -- not directly, as they are deprived of the Beatific Vision until they enter Heaven, but by infusing this knowledge into their souls. St. Bellarmine teaches that because the Church Suffering is so close to God -- much closer than we are and having the great consolation of knowing they are saved -- their prayers for us are very effective. So, as you pray for your dead loved ones, ask them to pray for you, too!
As to the damned, there is no hope; no prayer can help them and we can't pray formally for those in Hell. The problem, of course, is that we can't know who is damned, and so we pray generally for "all the faithful departed." For those who've died outside of visible Communion with Christ's Church or for those Catholics who've died seemingly without repentance and in scandal, public prayer cannot be offered, but we can most certainly still pray privately with the hope that they've died in a state of grace (i.e., those who are denied a Catholic funeral can't be prayed for liturgically, publicly, but they can most definitely be prayed for -- and should be prayed for -- privately). Priests can even offer Masses for such people privately, without naming them.
With regard to saying Masses for the Dead, the principle is the same. Christ has given us no greater means of prayer than the sublime act of Divine Worship itself. As the graces which flow from the Holy Sacrifice of the Mass are infinite, they do immeasurable good in expiating the sins of the faithful departed, the same as that sacrifice which Judas Maccabeus had offered for his fallen soldiers.
So pray for the dead. They need our prayers as much as we rely on their prayers. If you have loved ones whom you wish to be prayed for by many people, I recommend enrolling them in the Rorate Caeli Purgatorial Society that is periodically updated and that has a number of priests offering weekly and monthly Masses for all the souls listed, as well as those who make the request.
** Requiem aeternam dona eis, Domine, et lux perpetua luceat eis. Requiescant in pace. **