Purgatory, far from being a “Catholic invention” as most Protestants assume, was believed from the very beginning by Christians.
“According to the grace of God given to me, like a skilled master builder I laid a foundation, and another man is building upon it. Let each man take care how he builds upon it. For no other foundation can any one lay than that which is laid, which is Jesus Christ. Now if any one builds on the foundation with gold, silver, precious stones, wood, hay, straw — each man’s work will become manifest; for the Day will disclose it, because it will be revealed with fire, and the fire will test what sort of work each one has done. If the work which any man has built on the foundation survives, he will receive a reward. If any man’s work is burned up, he will suffer loss, though he himself will be saved, but only as through fire”. 1 Corinthians 3:10-15
This purification purges away all the dross that clings to the soul, things that St. Paul describes metaphorically as flammable materials such as “wood, hay, and straw.” These things are burned away in this judgment by God. Conversely, that man’s good works — which St. Paul compares with “gold, silver, and precious stones” — are refined and retained.
Since there are only two ultimate destinies possible for all human beings, heaven or hell, the issue of purgatory must be understood as simply a part of the process for some souls who are destined for heaven.
In Matthew 12:32, the Lord mentions a sin that cannot be forgiven even “in the world to come,” implying that there are some sins that will be forgiven after death.
Similarly, in the teaching about the Unforgiving Servant, Christ concludes with the fact that the wicked servant, even after his debt was canceled by the king, was thrown into prison for maltreating his fellow servant and told, “You wicked servant! I forgave you all that debt because you besought me; and should not you have had mercy on your fellow servant, as I had mercy on you?’ And in anger his lord delivered him to the jailers, till he should pay all his debt”. Matthew 18:32-34
Echoing this theme, St. Peter speaks about the souls who are “in prison,” awaiting their entrance into heaven 1 Peter 3:18-19, 4:6.