Let us go directly to the Catechism.
III. THE FINAL PURIFICATION, OR PURGATORY
1030 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.
1031 The Church gives the name Purgatory to this final purification of the elect, which is entirely different from the punishment of the damned.606 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:607
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.608
1032 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.”609 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.610 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.611
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The purification of the individual is based upon the Scriptural declaration that nothing that is not pure will enter the Kingdom of Heaven. Therefore, the sinful who are Judged to Salvation will have to undergo some sort of purification in order to become pure.
Catechism: “1030 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.”
Scripture:
“14For by one offering He has perfected for all time those who are sanctified.” - Hebrews 10
“6For I am confident of this very thing, that He who began a good work in you will perfect it until the day of Christ Jesus.” - Phil 1
“17By this, love is perfected with us, so that we may have confidence in the day of judgment; because as He is, so also are we in this world. 18There is no fear in love; but perfect love casts out fear, because fear involves punishment, and the one who fears is not perfected in love.” - 1 John 4
“4 But God, being rich in mercy, because of His great love with which He loved us, 5 even when we were dead in our transgressions, made us alive together with Christ (by grace you have been saved), 6 and raised us up with Him, and seated us with Him in the heavenly places in Christ Jesus...” - Ephesians 2
Heaven isn’t in the future. It starts now.
"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, paragraph 1030).
"The truth has been divinely revealed that sins are followed by punishments. God's holiness and justice inflict them. Sins must be expiated. This may be done on this earth through the sorrows, miseries and trials of this life and, above all, through death. Otherwise the expiation must be made in the next life through fire and torments or purifying punishments." (The Second Vatican Council, p. 63).
“The poor souls in purgatory cannot do anything for themselves to shorten their suffering. But Christ, their Redeemer, is always speaking to the Father for them, and Mary and the other Saints are also praying through Christ and with Christ.” (A Catholic Catechism, p. 337)
"An indulgence is obtained through the Church who, by virtue of the power of binding and loosing granted her by Christ Jesus, intervenes in favor of individual Christians and opens for them the treasury of the merits of Christ and the saints to obtain from the Father of mercies the remission of the temporal punishment due for their sins." The Church does this not just to aid Christians, "but also to spur them to works of devotion, penance, and charity" (CCC 1478).
Assisting with devotion at the procession of the holy Rosary (7 years and 7 quarantines of indulgence); Or "with faith, piety and love" saying "My lord and my God" at the elevation of the host during Mass (7 years); Kissing the Pope's (300-day indulgence, but a bishop's gets only 50); Ascending the holy stairs in Rome on one's knees, "whilst meditating on the passion of our Lord Jesus Christ" (9 years per step). - The Raccolta : or, Collection of indulgenced prayers and good works"
Key texts in opposition:
(Luke 23:43) "And Jesus said unto him, Verily I say unto thee, To day shalt thou be with me in paradise."
(2 Cor 5:8) "We are confident, I say, and willing rather to be absent from the body, and to be present with the Lord."
(1 Th 4:17) "Then we which are alive and remain shall be caught up together with them in the clouds, to meet the Lord in the air: and so shall we ever be with the Lord."
1. We are forgiven of all sins when we become born again (Col. 2:13), and that for those who die in the faith there is no further punishment, except the loss of rewards. The only postmortem suffering for saints is that they will suffer loss by their works being burned up, works which were done out in union with God and not led by His Spirit (1Cor. 3:10-15). But it is clear here that it is not the person who is affected by the fire but his works.
2. All the verses which clearly speak of a N.T. believer's postmortem condition (Luke 23:43; Acts 7:59; 1Cor. 15:52; 2 Cor 5:8; 1 Th 4:17; 1Jn. 3:2) show it is with the Lord, in whose presence there is fulness of joy (Ps. 16:11).
3. The Bible states that it is the chastening of the Lord in this life that works to make us holy (Heb. 12), and keep or bring us back to saving faith, that we be not "condemned with the world" (1Cor. 11:34), and thus judgment begins at the house of God (1Pt. 4:17). And in such texts that deal with the issue of chastening, nothing is even intimated of a postmortem period of such for New Testament believers, though different degrees of authority and glory (Mt. 13:43; 16:27; 19:28; Mk. 10:40; 1Cor. 15:41; Rv. 5:4; 20:4) seems evident, in accordance with every man being rewarded according to his own labor in the Lord (1Cor. 3:8).
4. It is the flesh in which no good thing dwells (Rm. 7:18) and which cannot be made subject to the law of God (Rm. 8:7) that is the problem, but which is not going to be purified, but is to be crucified (Rm. 6).
In the case of the penitent thief, surely are not to suppose that a few hours on the cross was what purified this man enough that he could go directly with God? Rather, his cry for salvation, out of a broken heart and a poor and contrite spirit (Ps. 34:18) brought him salvation, and the imputed righteousness of Christ *Rm. 4 ― 5:1). And if 1 Cor. 5:8 only refers to Paul and “canonized saints,” then surely he would have made a distinction between himself and other, less sanctified saints [all believers in the New Testament are “called saints,” Rm. 1:7; 1Cor. 1:2 (literally)]. But he tells the Corinthians, who needed the most work, that if the rapture occurred then they would go to be the Lord as well (1Cor. 15). As concerns the latter, the idea that souls who die in grace must be further purified before they can dwell with God militates against the incorruptible condition that raptured saints obtain, "For this corruptible must put on incorruption, and this mortal must put on immortality" (1 Cor 15:53). And since we shall be like Jesus when He shall appear (1Jn. 3:2), and we enter into the "glorious liberty of the children of God" (Rm. 8:21), then not only would saints who die before that time be yet absent from the LORD, but a radical reworking of eschatology would be needed, that would put the coming of the Lord after all the saints, dead and alive, have gone through an indeterminate time of purifying. Yet the Bible speaks of going immediately to be with God and the rapture as an event that could be imminent whenever the conditions were met (2Thes. 2:1-4).