According to the Catechism of the Catholic Church, an indulgence is “the remission before God of the temporal punishment due to sin whose guilt has already been forgiven. A properly disposed member of the Christian faithful can obtain an indulgence under prescribed conditions through the help of the Church which, as the minister of redemption, dispenses and applies with authority the treasury of the satisfactions of Christ and the saints. An indulgence is partial if it removes part of the temporal punishment due to sin, or plenary if it removes all punishment.”
Understanding the Catholic definitions is very important in understanding this issue: Eternal Punishment: “the penalty for unrepented mortal sin, separating the sinner from communion with God for all eternity; the condemnation of the unrepentant sinner to hell.” Temporal Punishment: “purification of the unhealthy attachment to creatures, which is a consequence of sin that perdures even after death. We must be purified either during our earthly life through prayer and a conversion which comes from fervent charity, or after death in purgatory.” Purgatory: “a state of final purification after death and before entrance into heaven for those who died in God’s friendship, but were only imperfectly purified; a final cleansing of human imperfection before one is able to enter the joy of heaven.”
The Roman Catholic Church teaches that sin has a double consequence. For a member of the Catholic Church, committing a mortal sin causes “eternal punishment,” involving eternal separation from God and suffering in hell. (The Catholic Church also teaches that under normal circumstances those who have not been baptized by either the Roman Catholic Church or another church teaching baptismal regeneration are also condemned to hell because the stain of original sin remains upon their souls.) Venial (minor) sin, in contrast, does not cause “eternal punishment” but does cause “temporal punishment.” Roman Catholic teachings sometimes refer to these “temporal punishments” given by God as a means of purifying His children (either in this life or in Purgatory). But the Roman Catholic Church also sees venial sins as creating a debt to God’s justice that must be atoned for in a way that is distinct from Christ’s atonement for eternal punishment. The Roman Catholic Church teaches that because of the unity of the Body of Christ (the Communion of the Saints, including living believers, believers in heaven, Roman Catholic saints in heaven, Christ, Mary, and the imperfect believers in Purgatory), it is possible for the merit generated by the good works, prayers, almsgiving, sufferings, etc., of one or more of these members of the Body to be applied to the temporal debt of another. The Roman Catholic Church teaches that the combined merit of Christ, the saints, and godly believers is stored in a place referred to as the Treasury of Merit (it is also sometimes called the Treasury of Satisfaction, the Church’s Treasury, or the Thesaurus Ecclesiae). And through apostolic succession from Peter, it is the Roman Catholic Church alone that has the authority to withdraw merit from this treasury and dispense it to believers in this life or in Purgatory to atone for some or all of their venial sin. This it does through the granting of Catholic indulgences.
Again, indulgences pertain only to temporal, not eternal, punishment and can only be distributed through a Roman Catholic Church leader to someone who is either in Purgatory or is still living and whose soul is in the state of sanctifying grace (i.e., he/she would go to Purgatory, not hell, if he/she were to die at that moment). An indulgence can be obtained through a good deed done, a Mass being offered on behalf of someone, prayer, abstinence, giving to the poor, or some other meritorious act performed in accordance with requirements set by a Pope or bishop having jurisdiction over that individual. The offering of a Mass for someone is seen as one of the most effective means of reducing the temporal punishment of that person in Purgatory. A partial indulgence will reduce the temporal punishment a person has. A plenary indulgence will remove all temporal punishment.
But the question is, is it biblically justified?