Here's a little Theology of Indulgences 101 according to the teaching of the Roman Catholic Church:
Indulgences are not for the forgiveness of sin but rather for the forgiveness of punishment.
There are two kinds of sin: (1) A mortal sin is a grave sin that leads to eternal punishment and exclusion from eternal life. (2) All other sins require temporal punishment, and the purification happens either (a) here on earth, or (b) after death in Purgatory. The punishment is not conceived of as vengeance from God from outside of us, but more like the natural consequence of the nature of sin itself.
So there are Christians in three locations: on earth, in Purgatory, and in heaven. All of them are united to Christ, and there is a mutual exchange of all good things so that the holiness of one can profit the holiness of someone else. This collective spiritual good is called Christ's Treasury--all of Christ's infinite merits before God, plus Mary's prayers and good works, plus the prayers and good works of all the saints in heaven. By relying on Christ's Treasury the purification from punishment can happen more quickly and effectively.
Because of its authority under Christ, the Catholic Church believes that it may open and administer the benefits of Christ's Treasury to Christians under their jurisdiction in response to their prayers and good works.
The Catholic Church has no jurisdiction over Christians in Purgatory, but Christians on earth can help them by obtaining indulgences for them through intercession.
There are two kinds of indulgences: (1) plenary indulgences remit all of the temporal punishment; (2) partial indulgences remit part of it.
How do you get a plenary indulgence? You must:
- abandon all attachment to all kinds of sin
- perform the work or say the prayer for which the indulgence is granted
- participate in sacramental confession
- participate in the Eucharist
- pray for the intentions of the Pope
So what do you need to do to get a partial indulgence? At minimum you must be contrite in your heart and perform the work or say the prayer for which the indulgence is granted.
It should also be said that the Church has clarified that people are not to seek indulgences merely to remit temporal punishment, but also as something that will spur them on to greater love.When it's all said and done, the whole idea of indulgences comes down to the idea that Christians must undergo a process of temporal punishment before they are allowed to go to heaven. But Scripture doesn't teach that. Rather, the punishment process was completely absorbed for us by Christ on the cross. We may receive discipline from our Father, but we will never again receive any form of punishment. Scripture no where hints that those who are united to Christ are in two categories and locations: those undergoing purificatory punishment in Purgatory and those in their heavenly home. All who die in Christ go to be with the Lord. Thanks be to God who clothes us with his righteousness and took all of our punishment.