Mere reason suggests there must be a Purgatory. So many people seem to be good, but not so greatly good that they should be fit for heaven at once. Again, not nearly all are so evil as to deserve hell. So there should be a means of purification and paying the debt of temporal punishment for those not fit for hell, nor for heaven at once. (Of course Luther would say we can sin all we want and still go to heaven at once, if only we believe it is all covered by Christ's merits: Epistle 501 to Melanchthon).
There is not much in Scripture on Purgatory except that in Second Maccabees 12:45, Judas sends a collection to the Temple for those fallen in battle, found with amulets on, "that they might be freed from this sin." Luther saw so clearly that this referred to Purgatory--which he rejected--that he rejected this book too, declaring it not part of Scripture. Some have tried to see an implication of Purgatory in Matthew 12:32. There Jesus speaks of the sin against the Holy Spirit that will be forgiven "neither in this world nor in the next." But the expression quoted is known in Rabbinic literature, where it means merely "never." Still less could we deduce purgatory from First Corinthians 3:11-15. Paul means if the work of some Christian worker has been of such low quality that it burns down, he himself will be saved "as through fire." But the fire seems to mean the apocalyptic fire of the last day, not a fire of purgatory.
But our belief in Purgatory rests on the tradition and definitions of the Church, at the Councils of Lyons II, Florence, and Trent.
The essential, perhaps the only suffering of Purgatory is the loss of God--it is like what we described in speaking of hell, except that in Purgatory there is no despair, rather, great consolation from assurance of salvation. Is there also something like fire in Purgatory? A host of private apparitions say there is; the Church has never pronounced on it. In fact the Eastern part of the Catholic Church has no such tradition. Many theologians say the suffering is greater than anything on earth. Neither Scripture nor Tradition tells us if that be so.
We do know that the souls there cannot merit or help themselves in any way anymore, they can only suffer. We know we can by prayers and penances relieve them, and somehow, they are enabled to know it when we do that, and they pray for us. How long should we pray and sacrifice for a particular soul? We do not know. St. Augustine in his Confessions (9:13), written 10 to 15 years after the death of his mother, St. Monica, still asked for prayers for her. If we can believe the private apparitions, Purgatory may last the equivalent of many years (we speak thus, for there is no time in Purgatory). For certain, it is terribly wrong to virtually canonize a person at the funeral, as Protestants do under the influence of Luther's sad mistake. Sadly not a few Catholics are imitating them.
Taken from The Basic Catholic Catechism
PART FOUR: The Apostle's Creed VI-VIII
Seventh Article: "From thence He shall come to judge the living and the dead"
By William G. Most. (c)Copyright 1990 by William G. Most