Martin Luther (1483-1546) needs no formal introduction, being one of the most recognizable religious figures in history. Luther was a German priest and theologian whose writings and teachings sparked the Protestant Reformation. Born shortly after the invention of the printing press, Luther became one of Europe's best selling authors, along with others like Erasmus, and his theology was mass produced across Europe. He strongly disputed the claim that freedom from God's punishment of sin could be purchased with money, known as indulgences. He confronted indulgence salesman Johann Tetzel with his Ninety-Five Theses in 1517, one of the most important writings of all time. His refusal to retract all of his writings at the demand of Pope Leo X in 1520 and the Holy Roman Emperor Charles V at the Diet of Worms in 1521 resulted in his excommunication by the pope and condemnation as an outlaw by the emperor.
Luther, of course, did not back down. Although the story about him nailing 95 Theses to a church door is almost surely apocryphal, the Protestant Reformation permanently broke off Protestants from the Catholic Church, which would lead to religious turmoil and bloodshed for the next 2 centuries.
Luther taught that salvation is not earned by good deeds but received only as a free gift of God's grace through faith in Jesus Christ as redeemer from sin. His theology challenged the authority of the Pope of the Roman Catholic Church by teaching that the Bible is the only source of divinely revealed knowledge and opposed sacerdotalism by considering all baptized Christians to be a holy priesthood. Those who identify with Luther's teachings are called Lutherans.