Perhaps one of the most unique doctrines of Catholicism is that of purgatory.  Purgatory is a place of burning torment and suffering that follows physical death, but precedes heaven.  According to Catholic doctrine, the blood of Jesus cleanses us from original sin – that is, the sin inherent in humans that occurred via the fall in the Garden of Eden.  Each individual commits sins on top of the original sin, for which only the sinner may atone.  This is achieved partly through penance and adherence to the sacraments during their lifetime.  The remaining blemishes of sin must be cleansed through the fires of purgatory, prior to entering heaven.  One can think of it in terms of twisted halfway house where one must be sufficiently tortured before being allowed to matriculate into decent society.  The Catholic Encyclopedia defines purgatory this way:

 Purgatory (Lat., "purgare", to make clean, to purify) in accordance with Catholic teaching is a place or condition of temporal punishment for those who, departing this life in God's grace, are, not entirely free from venial faults, or have not fully paid the satisfaction due to their transgressions.

 All sins are not equal before God, nor dare anyone assert that the daily faults of human frailty will be punished with the same severity that is meted out to serious violation of God's law. On the other hand whosoever comes into God's presence must be perfectly pure for in the strictest sense His "eyes are too pure, to behold evil" (Hab., i, 13). For unrepented venial faults for the payment of temporal punishment due to sin at time of death, the Church has always taught the doctrine of purgatory.

The Catholic Church teaches that faithful Catholics should pray for their loved ones who are in purgatory, in order to hasten their purification and allow them to go on to Heaven sooner.   

The Catholic doctrine of purgatory supposes the fact that some die with smaller faults for which there was no true repentance, and also the fact that the temporal penalty due to sin is it times not wholly paid in this life. The proofs for the Catholic position, both in Scripture and in Tradition, are bound up also with the practice of praying for the dead. For why pray for the dead, if there be no belief in the power of prayer to afford solace to those who as yet are excluded from the sight of God? So true is this position that prayers for the dead and the existence of a place of purgation are mentioned in conjunction in the oldest passages of the Fathers, who allege reasons for succouring departed souls. Those who have opposed the doctrine of purgatory have confessed that prayers for the dead would be an unanswerable argument if the modern doctrine of a "particular judgment" had been received in the early ages.

The “proofs” used by the Catholic Church to support the doctrine of purgatory come from Catholic Tradition (the writings of the Popes and saints) as opposed to Biblical scripture.  This isn’t to say that they have not attempted to support the doctrine Biblically.  The following passage from the Catholic Encyclopedia provides what it describes as proof from the Old Testament of the Bible.  However, this “proof” comes from the Apocrypha.  As stated in another section, the Apocrypha, or “Deuterocanonical Books” are hotly contested and are only found in the Catholic and Eastern Orthodox Bibles.  Most Protestants and Jews have disregarded the Apocrypha as scripture, due to numerous chronological and other errors that cast doubt on divine inspiration.  During the Protestant reformation, Luther sided with Jerome, the translator of the Latin Vulgate, in deciding that the Apocryphal books should not be considered Scripture.  Jerome gave his support to the Israel/Palestine Jews who rejected the Apocrypha instead of the Hellenistic Jews who readily embraced these books.  While the inclusion of the Apocrypha had been decided at the Councils of Hippo and Carthage late in the fourth century, Luther's action caused the Roman Catholic Church to react by reaffirming the canonicity of the Apocrypha at the Council of Trent in 1546.  It is largely based on these books that the Catholic Church supports their peculiar doctrines, including the doctrine of purgatory.  The passages mentioned below are found in the Apocrypha:

 The tradition of the Jews is put forth with precision and clearness in II Maccabees. Judas, the commander of the forces of Israel, "making a gathering . . . sent twelve thousand drachmas of silver to Jerusalem for sacrifice to be offered for the sins of the dead, thinking well and religiously concerning the resurrection (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). And because he considered that they who had fallen asleep with godliness, had great grace laid up for them. "It is therefore a holy and wholesome thought to pray for the dead, that they may be loosed from sins" (II Mach., xii, 43-46). At the time of the Maccabees the leaders of the people of God had no hesitation in asserting the efficacy of prayers offered for the dead, in order that those who had departed this life might find pardon for their sins and the hope of eternal resurrection.

If you are searching through your Bible trying to find II Maccabees, you'd better have a Catholic Bible.  As part of the Apocrypha, it is not in most Protestant or Jewish Bibles.  The Catholics also use the following passages from the Bible to support the doctrine of purgatory, but if you read the passages carefully, you’ll find the Catholic interpretations to be a wide stretch of what is written: 

There are several passages in the New Testament that point to a process of purification after death. Thus, Jesus Christ declares (Matthew 12:32): "And whosoever shall speak a word against the Son of man, it shall be forgiven him: but he that shall speak against the Holy Ghost, it shall not be forgiven him, neither in this world, nor in the world to come." According to St. Isidore of Seville (Deord. creatur., c. xiv, n. 6) these words prove that in the next life "some sins wil be forgiven and purged away by a certain purifying fire." St. Augustine also argues "that some sinners are not forgiven either in this world or in the next would not be truly said unless there were other [sinners] who, though not forgiven in this world, are forgiven in the world to come" (De Civ. Dei, XXI, xxiv). The same interpretation is given by Gregory the Great (Dial., IV, xxxix); St. Bede (commentary on this text); St. Bernard (Sermo lxvi in Cantic., n. 11) and other eminent theological writers.

A further argument is supplied by St. Paul in I Cor., iii, 11-15: "For other foundation no man can lay, but that which is laid; which is Christ Jesus. Now if any man build upon this foundation, gold, silver, precious stones, wood, hay stubble: Every man's work shall be manifest; for the day of the Lord shall declare it, because it shall be revealed in fire; and the fire shall try every man's work, of what sort it is. If any man's work abide, which he hath built thereupon, he shall receive a reward. If any man's work burn, he shall suffer loss: but he himself shall be saved, yet so as by fire." While this passage presents considerable difficulty, it is regarded by many of the Fathers and theologians as evidence for the existence of an intermediate state in which the dross of lighter transgressions will be burnt away, and the soul thus purified will be saved.

I’ll admit I’m somewhat at a loss as to how best address their interpretation of the scripture above – mostly because I don’t know what logic they used to twist these meanings out of those verses.  As for the passage from Paul’s first letter to the Corinthians, one should back up and start on verse 10, where Paul says he laid a foundation as an expert builder, by the grace God gave him.  Gold, silver, and precious stones represent durable work that will stand the test of divine judgment.  Wood, hay, or straw denotes worthless work that will not stand the test, and are consumed by fire.  The work of some believers will stand the test while that of others will disappear – emphasizing the importance of teaching the pure word of God.  As for the last verse that reads that he shall escape, yet so as by fire.  In the New International Version of the Bible, it reads, “he himself will be saved, but only as one escaping through flames.”  The best interpretation indicates that the original language was a Greek proverbial phrase, which means, “by a narrow escape”.  A modern equivalent might be, “escaped by the skin of his teeth”.  Paul is really trying to drive home this point:  Okay, so you’ve been saved.  Great.  Wonderful.  What are you going to do with this opportunity?  Will you squander it, or will you live a life in service to your Lord? 

One of the stranger aspects of purgatory is the belief that one can pray to the souls of loved ones who are in purgatory, and ask for intercession.  Some believe that the souls in purgatory can pray for and intercede in the lives of the living.  Here is what the Catholic Encyclopedia has to say on the matter:

Do the souls in purgatory pray for us? May we call upon them in our needs? There is no decision of the Church on this subject, nor have the theologians pronounced with definiteness concerning the invocation of the souls in purgatory and their intercession for the living. In the ancient liturgies there are no prayers of the Church directed to those who are still in purgatory. On the tombs of the early Christians nothing is more common than a prayer or a supplication asking the departed to intercede with God for surviving friends, but these inscriptions seem always to suppose that the departed one is already with God. St. Thomas (II-II:83:11) denies that the souls in purgatory pray for the living, and states they are not in a position to pray for us, rather we must make intercession for them. Despite the authority of St. Thomas, many renowned theologians hold that the souls in purgatory really pray for us, and that we may invoke their aid. Bellarmine (De Purgatorio, lib. II, xv,) says the reason alleged by St. Thomas is not at all convincing, and holds that in virtue of their greater love of God and their union with Him their prayers may have great intercessory power, for they are really superior to us in love of God, and in intimacy of union with Him. Suarez (De poenit., disp. xlvii, s. 2, n. 9) goes farther and asserts "that the souls in purgatory are holy, are dear to God, love us with a true love and are mindful of our wants; that they know in a general way our necessities and our dangers, and how great is our need of Divine help and divine grace".

When there is question of invoking the prayers of those in purgatory, Bellarmine (loc. cit.) says it is superfluous, ordinarily speaking, for they are ignorant of our circumstances and condition. This is at variance with the opinion of Suarez, who admits knowledge at least in a general way, also with the opinions of many modern theologians who point to the practice now common with almost all the faithful of addressing their prayers and petitions for help to those who are still in a place of purgation. Scavini (Theol. Moral., XI, n. l74) sees no reason why the souls detained in purgatory may not pray for us, even as we pray for one another. He asserts that this practice has become common at Rome, and that it has the great name of St. Alphonsus in its favour. St. Alphonsus in his work the "Great Means of Salvation", chap. I, III, 2, after quoting Sylvius, Gotti, Lessius, and Medina as favourable to his opinion, concludes: "so the souls in purgatory, being beloved by God and confirmed in grace, have absolutely no impediment to prevent them from praying for us. Still the Church does not invoke them or implore their intercession, because ordinarily they have no cognizance of our prayers. But we may piously believe that God makes our prayers known to them". He alleges also the authority of St. Catharine of Bologna who "whenever she desired any favour had recourse to the souls in purgatory, and was immediately heard".

In summary, the doctrine of purgatory is unbiblical.  Its foundation lies not in Biblical scripture, but in Catholic Tradition.  The Apocrypha should not be considered part of Biblical scripture, and the verses in the New Testament were twisted to fit an already established doctrine.