Why did medieval Christians believe that forgiven sins could still require satisfaction after death?

In medieval Roman Catholic theology, the belief that forgiven sins could still require satisfaction after death was rooted in the distinction between the guilt of sin and its temporal punishment. While the sacrament of penance provided absolution for the guilt (culpa) of sin, it was held that the offender still owed a temporal punishment (poena) to God for the offense committed.1

Scholastic theologians of the 12th and 13th centuries articulated a theory of penance consisting of three parts: contrition, confession, and satisfaction. Because this temporal debt might not be fully "satisfied" or expiated during one's earthly life through good works—such as charity, pilgrimage, or fasting—it was believed that the remaining debt had to be cleansed in purgatory. This state of purification allowed souls to balance their accounts and heal from accumulated defilements before entering heaven. Consequently, the system of indulgences and intercessory prayers was developed to help the faithful measure and amortize this remaining debt.123