Protestant
What is purgatory and is it biblical?
Purgatory is the final purification of those who die in God's friendship but are not yet perfectly free of sin's effects; Scripture speaks of a cleansing 'as through fire' and prayer for the dead, both pointing to this reality.
Catholic teaching
Nothing unclean can enter heaven, so souls who die imperfectly purified undergo a purification after death that is not a 'second chance' but the completion of the sanctification already begun by grace, aided by the prayers of the living.
Biblical foundation
Paul describes a man saved 'but only as through fire' after his works are tested (1 Cor 3:12-15), and Judas Maccabeus 'made atonement for the dead, that they might be delivered from their sin' (2 Macc 12:44-46), presupposing purification after death.
Scripture references
Historical evidence
Early Christian funerary inscriptions and liturgies from at least the second century include prayers for the repose of the dead, a practice that makes sense only if some purification after death was believed possible.
The Church Fathers
“I pray thee for the sins of my mother... and let none separate her from thy protection.”