Private Revelation on Purgatory:
Jesus talks about purgatory in Matthew 5:26, when He says ‘Truly I tell you, you will not get out until you have paid back the last penny’ (atonement for our sins). Our Lord to St. Gertrude “My love urges Me to release the poor souls. If a beneficent king leaves his guilty friend in prison for justice’s sake, he awaits with longing for one of his nobles to plead for the prisoner and to offer something for his release. Then the king joyfully sets him free. Similarly, I accept with highest pleasure what is offered to Me for the poor souls, for I long inexpressibly to have near Me those for whom I paid so great a price.” Most souls, even those who went to Confession during their life or before death were forgiven their sins but they have not always atoned for their sins. As our sins have great effect on ourselves and others. Having Mass said for a soul is a way to help the soul purify their sins and receive great relief and ascend to a next level of purgatory. A purgatorial soul was asked what the most effective deliverance to Heaven is for a soul and they said ‘Mass, and after that the Way off the Cross..’.
St. Paul talks about the Fires of Purgatory:
1 Corinthians 3:10-15 According to the grace of God given to me, like a skilled master builder I laid a foundation, and someone else is building on it. Each builder must choose with care how to build on it. For no one can lay any foundation other than the one that has been laid; that foundation is Jesus Christ. Now if anyone builds on the foundation with gold, silver, precious stones, wood, hay, straw—the work of each builder will become visible, for the Day will disclose it, because it will be revealed with fire, and the fire will test what sort of work each has done. If what has been built on the foundation survives, the builder will receive a reward. If the work is burned up, the builder will suffer loss; the builder will be saved, but only as through fire.