True indulgences existed since the beginning of the Christian (Catholic) Church and the Church continues to grant them today.
Thanks to forgers who were working for disobedient Bishops during the Middle ages, the Catholic Church inherited the label of "Selling indulgences." These forgers would write fake ingulgences offering for the forginess of sins in exchange for money which was often used for the construction of Catholic Churches.
It took the Popes of those days three centuries to end the abuse of the sale of indulgences. Unfortunately, there are some who love to condemn innocent people for the sins of their ancestors. Today's Catholics are not to be blamed for the sins of corrupt Bishops and forgers of the past.
A BRIEF EXPLANATION OF INDULGENCES
Why were Catholics buying indulgences in the Middle ages? To understand that, it is necessary to understand what an indulgence is.
According to Catholic teachings, when you sin, you get two punishments. You get:
1. An eternal punishment (hell):
To be free of the eternal punishment, one must receive the Sacrament of Confession and be forgiven of his sin(s).
2. A temporal punishment on earth while you are alive, (or in purgatory after death):
To be free of one's temporal punishment, a person must receive an indulgence. An indulgence is a special "blessing" that the sinner receives when he performs a special act such as doing a good deed or saying certain prayers. Upon the completion of the conditions of the indulgence, the person's temporal punishment is removed.
So, why were Catholics buying indulgences in the Middle ages? They believed this would remove the temporal punishment attached to their sins. By paying for the indulgences, they would no longer have to perform special acts of good deeds or say certain prayers and still, they would go to Heaven without having to go to Purgatory.