Historically significant because it was a catalyst that led to the Protestant Reformation, the concept of indulgences is a teaching of Catholic theology that states, essentially, remission of sin is granted by the Church based on the sinner's actions, following absolution of sin through the sacrament of Confession. Opposition to the corrupt practice by the Church of selling indulgences was one of the primary tenets of Martin Luther's Ninety-Five Theses; he countered that forgiveness of sin is granted by God alone, not through intermediaries, and through God's grace rather than the works of individuals.
According to the Catechism of the Catholic Church, two types of sin exist: A mortal sin is a grievous wrongdoing, committed willfully and knowingly, which causes the soul to become dead, thus condemning the individual to immediately enter Hell upon death (although repentance can mitigate this sentence). Venial ("forgivable") sins are lesser wrongdoings not committed deliberately or with full knowledge, which lead to a "partial loss of grace" from God and weakening of one's soul, leading to a separation from God.
Indulgences are the full or partial remission of punishment due for sins that have been forgiven, granted by the Church after a Catholic receives absolution through the sacrament of confession. The Catholic Church teaches that indulgences come from the "Treasury of Merit," which is a direct benefit of Christ's death on the cross as well as the holiness, or virtues and penances, of the Communion of Saints (all humans who have died having all mortal sins forgiven). Indulgences are granted for specific good works and prayers, which can include intercession by others, and remit all or part of the temporal punishment meted out for the sin.
The early Church had strict rules governing penances were given before indulgence was granted, which were often extremely harsh. Like any commodity that has value, indulgences were abused and sold. The corruption within the Church led to the separation that eventually became the Protestant Reformation. Some religious philosophers believe that the concept of any humans having the authority to forgive sins and granting indulgences is a tool that controls the behavior of people under the Church's jurisdiction, and the thought of living in sin creates psychological and emotional issues that make the individual more able to be controlled.
According to the Catechism, plenary (complete) indulgence is granted when all attachments to any kind of sin are excluded, the work and/or prayers on which the indulgence is granted are completed, and three conditions are fulfilled: sacramental confession, a Eucharistic communion, and praying for the intentions of the pope. Partial indulgence can be granted if one is contrite in heart, meaning the Catholic has completed the work or prayers or another has done so on behalf of the individual. In 1967, Pope Paul VI revised the specifics pertaining to indulgences in the Indulgentiarum doctrina.
In some locations the Eastern Orthodox Church also incorporated a form of indulgences, as absolution certificates issued by patriarchs and distributed solely by the Greek Orthodox Patriarch of Jerusalem, prior to the 20th century. This theology teaches that sins can be absolved by the Mystery of Confession, preceded by fasting in eastern locations.