Matt. 5:48 - Jesus says, “be perfect, even as your heavenly Father is perfect.” We are only made perfect through purification, and in Catholic teaching, this purification, if not completed on earth, is continued in a transitional state we call purgatory.
Matt. 12:32 – Jesus says, “And anyone who says a word against the Son of man will be forgiven; but no one who speaks against the Holy Spirit will be forgiven either in this world or in the next.” Jesus thus clearly provides that there is forgiveness after death. The phrase “in the next” (from the Greek “en to mellonti”) generally refers to the afterlife (see, for example, Mark 10.30; Luke 18.30; 20.34-35; Eph. 1.21 for similar language). Forgiveness is not necessary in heaven, and there is no forgiveness in hell. This proves that there is another state after death, and the Church for 2,000 years has called this state purgatory.
Luke 12:47-48 - when the Master comes (at the end of time), some will receive light or heavy beatings but will live. This state is not heaven or hell, because in heaven there are no beatings, and in hell we will no longer live with the Master.
Luke 16:19-31 - in this story, we see that the dead rich man is suffering but still feels compassion for his brothers and wants to warn them of his place of suffering. But there is no suffering in heaven or compassion in hell because compassion is a grace from God and those in hell are deprived from God’s graces for all eternity. So where is the rich man? He is in purgatory.
1 Cor. 15:29-30 - Paul mentions people being baptized on behalf of the dead, in the context of atoning for their sins (people are baptized on the dead’s behalf so the dead can be raised). These people cannot be in heaven because they are still with sin, but they also cannot be in hell because their sins can no longer be atoned for. They are in purgatory. These verses directly correspond to 2 Macc. 12:44-45 which also shows specific prayers for the dead, so that they may be forgiven of their sin.
Phil. 2:10 - every knee bends to Jesus, in heaven, on earth, and “under the earth” which is the realm of the righteous dead, or purgatory.
2 Tim. 1:16-18 - Onesiphorus is dead but Paul asks for mercy on him “on that day.” Paul’s use of “that day” demonstrates its eschatological usage (see, for example, Rom. 2.5,16; 1 Cor. 1.8; 3.13; 5.5; 2 Cor. 1.14; Phil. 1.6,10; 2.16; 1 Thess. 5.2,4,5,8; 2 Thess. 2.2,3; 2 Tim. 4.8). Of course, there is no need for mercy in heaven, and there is no mercy given in hell. Where is Onesiphorus? He is in purgatory.
Heb. 12:14 - without holiness no one will see the Lord. We need final sanctification to attain true holiness before God, and this process occurs during our lives and, if not completed during our lives, in the transitional state of purgatory.
Heb. 12:23 - the spirits of just men who died in godliness are “made” perfect. They do not necessarily arrive perfect. They are made perfect after their death. But those in heaven are already perfect, and those in hell can no longer be made perfect. These spirits are in purgatory.
1 Peter 3:19; 4:6 - Jesus preached to the spirits in the “prison.” These are the righteous souls being purified for the beatific vision.
Rev. 21:4 - God shall wipe away their tears, and there will be no mourning or pain, but only after the coming of the new heaven and the passing away of the current heaven and earth. Note the elimination of tears and pain only occurs at the end of time. But there is no morning or pain in heaven, and God will not wipe away their tears in hell. These are the souls experiencing purgatory.
Rev. 21:27 - nothing unclean shall enter heaven. The word “unclean” comes from the Greek word “koinon” which refers to a spiritual corruption. Even the propensity to sin is spiritually corrupt, or considered unclean, and must be purified before entering heaven. It is amazing how many Protestants do not want to believe in purgatory. Purgatory exists because of the mercy of God. If there were no purgatory, this would also likely mean no salvation for most people. God is merciful indeed.
Gen. 50:10; Num. 20:29; Deut. 34:8 - here are some examples of ritual prayer and penitent mourning for the dead for specific periods of time. The Jewish understanding of these practices was that the prayers freed the souls from their painful state of purification, and expedited their journey to God.
Zech. 9:11 - God, through the blood of His covenant, will set those free from the waterless pit, a spiritual abode of suffering which the Church calls purgatory.