In the Roman Catholic Church this is the remission of all or part of the debt of temporal punishment owed to God due to sin after the guilt has been forgiven. This grant is based on the principle of vicarious satisfaction, which means that since the sinner is unable to do sufficient penance to expiate all his sins, he is able to draw on the spiritual treasury formed by the surplus merits of Christ, the Virgin Mary, and the saints. The authority for granting indulgences rests with the pope, although he may designate others (e.g., cardinals, bishops) to have this power, with the exception of indulgences for the dead.
Most indulgences granted by the pope are applicable to the souls in purgatory, Apostolic indulgences are those attached to religious articles (such as crucifixes, statues, medals, and rosaries), or to the performance of certain works on special feast days, or the recitation of certain holy names at the hour of death. Other indulgences may be gained by fulfilling the prescribed conditions and completing a designated act (usually the recitation of certain prayers) in the manner specified by the granting authority. To gain an indulgence it is necessary to be a member of the Catholic Church, in a state of grace, and to have the intention of gaining the indulgences. Other conditions, such as confession and Communion, may be required in some instances. A plenary indulgence remits the entire payment of punishment due up to the point when it is gained, while a partial indulgence remits only part of the punishment.
Although instances of forms of indulgences, such as commutations of penance and absolution grants, can be found in the early church, it was not until the eleventh century that indulgence grants appeared which relaxed penitential acts on the condition that contributions be made to a church or monastery. The practice of granting indulgences became more widespread with the advent of the Crusades, beginning with the First Crusade in 1095 when Urban II promised the remission of all penance to those who set out to liberate the Holy Land. Later this was extended to a plenary indulgence and came to include those who contributed to the support of the Crusades. The abuse of the granting of indulgences in return for financial support was considerable during the, and it eventually touched off the Protestant Reformation when attacked the doctrine itself in his “Ninety-five Theses.”
E.J. Ross (tr.), Indulgences as a Social Factor in the Middle Ages (1922); W. Herbst, Indulgences (1955); P.F. Palmer (ed.), Sacraments and Forgiveness, vol. 2 of Sources of Christian Theology (2 vols., 1955- 60).