Indulgences
Originally, indulgences were objects taken from the tombs of martyrs (or the prisons of imprisoned Christians expected to become martyrs), and, therefore, not unlike relics. They were presented by lapsed Christians to bishops to prove they were sincere in wanting to be readmitted into the Christian fold. This practice was recounted and encouraged by Saint Cyprian, who was Bishop of Carthage during the reign of the Emperor Decius. During the latter's brief reign (249-251 AD), Christians throughout the Roman Empire were persecuted. He had bishops killed, and used force or threat of force to compel Christian laymen to revert to the religions of their ancestors. Christians who had resisted pressure to renounce their faith found it difficult to accept former apostates who wanted to be reconciled. The antipope Novation objected to readmitting them.
During the early medieval period, the indulgence evolved into a substitution for an act of penance. For instance, at the Irish Synod of 807 AD, it was agreed the act of reciting the palms or the giving of alms to a poor person could be substituted for fasting. Pilgrimages to St. Albans in England, Compestela in Spain, Jerusalem, or Rome were made in lieu of penance.
By earning a plenary indulgence, one gains for oneself remission from future punishment in Purgatory for a sin for which one has already confessed to a priest, received absolution, and, if one victimzed someone else, made amends with whomever one has wronged.