It is said (2 Maccabees 12:46): “It is a holy and wholesome thought to pray for the dead, that they may be loosed from sins.” Now there is no need to pray for the dead who are in heaven, for they are in no need; nor again for those who are in hell, because they cannot be loosed from sins. Therefore after this life, there are some not yet loosed from sins, who can be loosed therefrom; and the like have charity, without which sins cannot be loosed, for “charity covereth all sins” [Proverbs 10:12]. Hence they will not be consigned to everlasting death, since “he that liveth and believeth in Me, shall not die for ever” [John 11:26]: nor will they obtain glory without being cleansed, because nothing unclean shall obtain it, as stated in the last chapter of the Apocalypse (verse 14). Therefore some kind of cleansing remains after this life.

Further, Gregory of Nyssa [De iis qui in fide dormiunt] says: “If one who loves and believes in Christ,” has failed to wash away his sins in this life, “he is set free after death by the fire of Purgatory.” Therefore there remains some kind of cleansing after this life.

From the conclusions we have drawn above (III, 86, 4-5; Supplement, 12, 1) it is sufficiently clear that there is a Purgatory after this life. For if the debt of punishment is not paid in full after the stain of sin has been washed away by contrition, nor again are venial sins always removed when mortal sins are remitted, and if justice demands that sin be set in order by due punishment, it follows that one who after contrition for his fault and after being absolved, dies before making due satisfaction, is punished after this life. Wherefore those who deny Purgatory speak against the justice of God: for which reason such a statement is erroneous and contrary to faith. Hence Gregory of Nyssa, after the words quoted above, adds: “This we preach, holding to the teaching of truth, and this is our belief; this the universal Church holds, by praying for the dead that they may be loosed from sins.” This cannot be understood except as referring to Purgatory: and whosoever resists the authority of the Church, incurs the note of heresy. (ST Appendix II)