The concept long predates the Middle Ages. In 2 Maccabees 12:46, Jews offer prayers and sacrifice for the dead so they might be freed from sin, a practice that only makes sense if purification after death is real. Jesus Himself speaks of a sin that 'will not be forgiven either in this age or in the age to come' (Matthew 12:32), implying some sins can be forgiven after death. The Catechism teaches that Purgatory is not punishment but purification: the final preparation of souls who died in God's friendship but still need to be made perfect for heaven (CCC 1030-1032).