This is a deep topic that cannot be adequately dealt with in a short article. Some additional sources are listed at the end of this article from the Hawaii Catholic Herald.
Many Protestants do not believe in purgatory, claiming that this teaching denies the sufficiency of Christ’s sacrifice for our sins. Some point, too, to the fact that the word “purgatory” exists nowhere in the Bible, but that argument seems to lack force. The words “Trinity” and “Incarnation” do not appear either.
Actually, a belief in a postmortem purification predates Christianity. Shortly before the advent of Christ, in the Old Testament Book of Second Maccabees (12:39-46), Judas Maccabeus was collecting the bodies of fallen comrades when he noticed that some of them had been wearing pagan amulets; so he arranged for an expiatory sacrifice and “made atonement for the dead that they might be absolved from their sin.” The implication, of course, is that some sins can be forgiven in the world to come.
That implication is confirmed in the Gospel of Matthew (12:32), where Jesus says, “Whoever speaks a word against the Son of Man will be forgiven; but whoever speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come.” The Fathers of the Church allude to the standard practice of praying for the dead. In the middle of the third century, St. Cyprian of Carthage told us that prayers for the departed had been said in all the churches since the time of the apostles.
This belief is codified in the Catechism of the Catholic Church: “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” (No. 1030-1031). What we don’t know, of course, is the exact nature of that purification, how many people undergo it, or whether it might even be instantaneous.
Read more in the “Why Do We Do That?” series from Deacon Mike Fritz.