Why did Martin Luther leave the Catholic Church?

Martin Luther, who lived from 1483 to 1546, was one of the champions of the Protestant Reformation. He was a German monk and professor of theology. Through his studies and work, he came to disagree with some of the ideas and practices of the Roman Catholic Church. He founded the Church of Lutheranism (another sect of Christianity) and taught that only the Bible may be considered an authority on the word of God. (This was a big snub to the Pope!) He wrote in his Ninety-Five Theses that the Catholic Church had become corrupt, primarily because of its practice of "indulgences." A person who committed sins could donate a sum of money to the Church in order for a priest to pray on their behalf. Essentially, Luther believed this meant people bought the forgiveness of God, and Luther disagreed with the idea that a person's redemption could be bought or earned through good deeds. Instead, Luther believed that the only source of salvation was faith in Jesus Christ. 

In 1521, Luther was officially excommunicated by the Catholic Church. Because his teachings were contrary to those of the Catholic Church, he had to go into hiding or face punishment. He continued to preach and write after excommunication, and in the following years his ideas and criticisms of the Church swept across Europe in the form of the Reformation.