Martín Luther was a German monk and theologian who lived in the 16th century. He is best known for initiating the Protestant Reformation, a movement that sought to reform the practices and beliefs of the Roman Catholic Church. Luther's famous act of posting his Ninety-Five Theses in 1517 criticized the sale of indulgences and other church practices, sparking widespread debate. Luther's ideas emphasized the importance of faith and the authority of the Bible over church traditions. His translation of the New Testament into German made the scriptures more accessible to the general public, significantly influencing Christianity and European society.