In the 14th century, the Catholic Church had what were known as 'indulgences,' in which you pay the church to absolve your sins. Martin Luther spoke out against the idea.
Also, Martin Luther was against it because he believed you couldn't buy your way into heaven, which was exactly how indulgences were being presented.
Also, poor people were starving themselves in order to afford them because they promise of eternal paradise seemed like a better investment than survival.
Side note that some very cool Catholic saints living at the time also recognized that it was a bad thing to do (there was a lot of corruption goin around) and they set about the Reformation, which fixed a lot of that during their lifetime. So there were a couple solutions in effect actually.
Fun fact: Martin Luther wasn't trying to start a new religion, he was trying to advocate reforms in the Catholic church. However due to his criticism of the church the church the church condemned him. And then he formed a new church with those who agreed with him. The Catholic reformation was started only after Martin Luther had founded protestantism. Presumably in part because the Pope at the time was not the same man who condemned Martin Luther. The Pope that condemned Martin Luther was dead before the Catholic reformation started.
Also, poor people were starving themselves in order to afford them because they promise of eternal paradise seemed like a better investment than survival.
Jokes apart, this is a very very bad thing to do.