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St. Paul taught purgatory most clearly.

[9] For we are God's coadjutors: you are God's husbandry; you are God's building. [10] According to the grace of God that is given to me, as a wise architect, I have laid the foundation; and another buildeth thereon. But let every man take heed how he buildeth thereupon. [11] For other foundation no man can lay, but that which is laid; which is Christ Jesus. [12] Now if any man build upon this foundation, gold, silver, precious stones, wood, hay, stubble: [13] 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. [14] If any man's work abide, which he hath built thereupon, he shall receive a reward. [15] If any man's work burn, he shall suffer loss; but he himself shall be saved, yet so as by fire.
(1 Cor. 3)

Observe: the inferior works of the believer are burned at his judgment, and he, purified, enters heaven. That is the essence of Purgatory.