Purgatory's meaning and origin

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.Catechism of the Catholic Church 1030

The Roman doctrine of Purgatory teaches that the elect (with few exceptions) are sent to a place of "temporal punishment" (Catechism, 1473). It is understood to have originated from ideas held since before the Early Middle Ages (AD 500) of purification after death. Jacques Le Goff dated the "birth" of Purgatory as being somewhere between AD 1000 and 1200. Augustine and Pope Gregory I (Gregory the Great) made notable contributions to Rome's understanding of Purgatory, which was stated doctrinally at the Coucils of Lyon (1245, 1274), Florence (1439), and Trent (1545-1563).

Eastern Orthodox belief is that when a person dies, their soul goes to Hades. They call their version of Purgatory "Particular Judgement," as the elect receive a foretaste of heaven, and the condemned receive a foretaste of their eternal suffering. With few exceptions, all the dead enter into this particular judgement and await the return and judgement of Christ, at which time they will be enter into their final reward or punishment.¹

Since Purgatory occurs after death, Catholics can lessen the time spent there by paying for their sins in this life through things such as indulgences or the reciting the Mass or the Rosary. Catholics also believe that the living should seek to intercede for the dead through indulgences and other means to shorten the time they must spend in Purgatory.

The Catholic justification

The Roman church attempts to justify the doctrine of Purgatory using Pope Gregory I's interpretation of Matthew 12:32.

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.Gregory I, quoted in Catechism of the Catholic Church 1031

Romanists also cite the Apocrypha, calling it "Sacred Scripture," which speaks of Judas Maccabeus "atonement for the dead, that they might be delivered from their sin" (quoted in Catechism 1032).

Refuting the Heresy

Simply put, Purgatory is not found anywhere in Scripture. The Catholic doctrine of Purgatory only makes sense to someone with a fundamental misunderstanding of the crucifixion of Christ. If Christ's sacrifice is sufficient for our sins (it is), if we are eternally secure (we are), if our debt is paid in full (it is), and if God accepts us not based on our own righteousness but on Christ's (He does), then Purgatory has no place in Christianity.

Regarding the verse in Matthew, it should be understood in context. While the meaning of the verse is disputed (even what exactly the sin is), it is not speaking about different ages, but about the finality of a certain sin. Also, forgiveness is given to us by grace through Christ in faith alone (Ephesians 2:8-9). Even if we were to allow the Roman interpretation of Matthew, allowing for forgiveness after death, that forgiveness would still come by grace through faith in Christ alone. Of course, Romanists would still have to show that the two ages mean life and death, rather than the more likely present age and kingdom age.

Notes

1 According to ORTHODOXY AND ROMAN CATHOLICISM, Michael Azkoul