What were some of the issues that led Martin Luther to break with the Catholic Church?

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As a Catholic priest, Martin Luther was greatly disturbed by the practice of selling indulgences; also, he was greatly troubled by the structure of the authority of the Church and the power that its hierarchy wielded.

Luther felt that the Church had become corrupt in its power. People were led to believe that if they contributed to the Church, their sins could be forgiven. Luther felt that the Pope had no power from God to absolve people from their sins simply because they transferred money to the Church. In essence, he felt that the Church was stealing from people through the selling of indulgences as well as by making them work on church grounds for free and by forcing them to tithe.

Luther was also disturbed by the hierarchical structure of the Catholic Church which gave great power to the cardinals and bishops over the other members of the clergy. In addition, because the Catholic Church was a political power as well as a religious one, cardinals often had significant roles in secular affairs, as well as within governments. 

Luther's 95 Theses that he nailed to a church door became famous and grew to be one of the most influential figures in European history. Certainly, his writings helped to cause friction within the Catholic Church and initiate the Protestant Reformation.