"Purgatory is based on the Catholic (not Biblical) teaching teach that some who sin and repent in this life may still have to undergo, after death, punishments from God for their transgressions.

Catholics believe that some people retain a "stain" on their souls for sins they have committed even if they repented of them and asked for forgiveness. When they die, according to their teachings, this stain remains on their immortal souls. Those whose stained souls are not considered worthy of eternal torment in hell fire must still have them "cleaned" before entering God's presence."

I remember my mom telling me as a kid that when I sin there is a stain on my soul. I believed that for years.