Purgatory

"... 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. Thomas Aquinas: Summa Theologica, Supplementum Tertiæ Partis (Appx. II), A. 1.

Also see (Appx. I) Q. 2.

Dante and the Three Kingdoms (detail), by Domenico di Michelino.
 Oil on canvas, 1465; Museo dell'Opera del Duomo, Florence.