What practice allowed people to gain forgiveness by giving the catholic church money?

The practice called an Indulgences.  Selling of forgiveness by the Catholic Church. It was common practice when the church needed to raise money. The practice led to the Reformation. Tetzel was one of the priest selling indulgences. Simony is what I called when a bishops selling positions in the Church. 

i r r e l e v a n t

it depends on which society you are referring to. many civilization performed rituals so the gods would with their growing season but the most well known is that of the aztecs. they would make sacrifices, usually animals, but sometimes humans as well.