Purgatory is the place and state in which the souls of the just who died in venial sin and with the debt of temporal punishment for grave sins remitted, are subjected to purifying sufferings until, having paid their debt, they are worthy of paradise. The existence of purgatory is a truth of faith defined by the Council of Trent.
Holy Scripture: “It is . . . a holy and wholesome thought to pray for the dead, that they may be loosed from sin” (2 Mach. 12:46). St. Paul (1 Corinthians 3:11ff) speaks of those who, having some remnants of sin mixed with good works, will be saved in the next life quasi per ignem (through fire).
Tradition: In the first centuries there was no EXPLICIT doctrine on purgatory, but they had the liturgical usage of prayers for the dead, reflected also in the epigraphy of the Catacombs. From the time of St. Augustine the doctrine of purgatory was developed, which continues substantially unchanged in the East and the West. The Scholastics treat of purgatory as of something belonging to the doctrine of faith. Luther and Calvin were wrong, therefore, in rejecting purgatory as a diabolic invention.
The Church, while defending the existence of purgatory, has not defined explicitly what its pains are: incidental mention is made of fire in I Council of Lyon, in a Letter of Clement VI. Certainly there is in purgatory a temporary pain of loss (poena damni - privation of the vision and possession of God), mitigated by the sure hope of entering paradise after due expiation. A pain of sense (poena sensus) commonly is admitted by the Fathers and theologians, fire not excluded. Purgatory will last only to the day of judgment.
Hope this helps! :tiphat: