2.35 What was the business with the Church selling indulgences as tickets to heaven?
God wants everyone to be able to go to heaven. Jesus instituted the Sacrament of Reconciliation (confession) so that we can receive forgiveness of our sins after Baptism. To be totally without (remnants of) sin upon arrival in heaven with God, these have to be expiated in purgatory.
Sometimes the pope can grant an indulgence to reduce the time spent in purgatory. This indulgence is a sign of God’s mercy and can be obtained, for example, by praying a certain prayer or by making a pilgrimage. In essence, therefore, indulgences are something beautiful. In the Middle Ages, however, indulgences were crudely abused and sold as “tickets to heaven”. Criticism of this practice is entirely justified.
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This is what the Popes say
A balanced and sound practice of gaining indulgences, whether for oneself or for the dead, can be helpful for a renewed appreciation of the relationship between the Eucharist and Reconciliation. By this means the faithful obtain "remission before God of the temporal punishment due to sins whose guilt has already been forgiven..." Since the conditions for gaining an indulgence include going to confession and receiving sacramental communion, this practice can effectively sustain the faithful on their journey of conversion and in rediscovering the centrality of the Eucharist in the Christian life. [Pope Benedict XVI, Sacramentum Caritatis, n. 21]