Why the topic of indulgence is closely related to luther's criticism of the catholic church

The topic of indulgence is closely related to Luther's criticism of the catholic church because of corruption within the church.

Explanation:

Indulgence is a feature of the sorrowful system of both the Western medieval and the Roman Catholic Church according to which full or partial cancellation of the sin’s punishment was granted.  

The pardoners used indulgences and misled churchgoers. It had become a major source of corruption there. They offered them forgiveness in exchange for money. Luther didn't like this fact that the people could buy indulgences and then get reduced punishment after death. This was misusing of faith of God in people. He believed Christians should believe only in Bible.

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