What is Purgatory?

"All who die in God's grace and friendship, but still imperfectly purified, are indeed assured of their eternal salvation; but after death they undergo purification, so as to achieve the holiness necessary to enter the joy of heaven. The Church gives the name Purgatory to this final purification of the elect, which is entirely different from the punishment of the damned. The Church formulated her doctrine of faith on Purgatory especially at the Councils of Florence and Trent. The tradition of the Church, by reference to certain texts of Scripture, speaks of a cleansing fire:

"As for certain lesser faults, we must believe that, before the Final Judgment, there is a purifying fire. He who is truth says that whoever utters blasphemy against the Holy Spirit will be pardoned neither in this age nor in the age to come. From this sentence we understand that certain offenses can be forgiven in this age, but certain others in the age to come.

"This teaching is also based on the practice of prayer for the dead, already mentioned in Sacred Scripture: 'Therefore [Judas Maccabeus] made atonement for the dead, that they might be delivered from their sin.' From the beginning the Church has honored the memory of the dead and offered prayers in suffrage for them, above all the Eucharistic sacrifice, so that, thus purified, they may attain the beatific vision of God. The Church also commends almsgiving, indulgences, and works of penance undertaken on behalf of the dead:

"Let us help and commemorate them. If Job's sons were purified by their father's sacrifice, why would we doubt that our offerings for the dead bring them some consolation? Let us not hesitate to help those who have died and to offer our prayers for them."

— From the Catechism of the Catholic Church, 1030-1032

Why are there souls in Purgatory?

"To understand this doctrine and practice of the Church, it is necessary to understand that sin has a double consequence. Grave sin deprives us of communion with God and therefore makes us incapable of eternal life, the privation of which is called the "eternal punishment" of sin. On the other hand every sin, even venial, entails an unhealthy attachment to creatures, which must be purified either here on earth, or after death in the state called Purgatory. This purification frees one from what is called the "temporal punishment" of sin. These two punishments must not be conceived of as a kind of vengeance inflicted by God from without, but as following from the very nature of sin. A conversion which proceeds from a fervent charity can attain the complete purification of the sinner in such a way that no punishment would remain."

— From the Catechism of the Catholic Church, 1472

Why Pray for the Holy Souls in Purgatory?

The souls in Purgatory are suffering real pain to cleanse them of the stain of their sins, both venial sins and forgiven mortal sins. One reason they feel pain is because they know that they have obtained salvation, but their enjoyment of Heaven is delayed. St. Thomas Aquinas says, "The more one longs for a thing, the more painful does deprivation of it become. And because after this life, the desire for God, the Supreme Good, is intense in the souls of the just (because this impetus toward him is not hampered by the weight of the body, and that time of enjoyment of the Perfect Good would have come) had there been no obstacle; the soul suffers enormously from the delay." These people suffering are relatives and friends, who will one day thank us for our prayers for them, and who will also pray for us.