Luther protested the practice of selling indulgences. what was that practice

Indulgences are sold by clergy that release people from performing penalties from their sins.  

Explanation:

Martin Luther King was the chief of Protestantism and social activist. He was Baptist minister. He was the youngest person who win the Nobel prize. From mid 1950 he played vital role in civil movement America.  

Martin Luther king has problem with teaching system of catholic and their way of justice.  That time anyone can purchased indulgences. He was against medieval penitential system.  

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