Martin Luther ( LOO-thər; German: [ˈmaʁtiːn ˈlʊtɐ] ; 10 November 1483 – 18 February 1546) was a German priest, theologian, author, hymnwriter, professor, and former Augustinian friar. Luther was the seminal figure of the Protestant Reformation, and his theological beliefs form the basis of Lutheranism. He is widely regarded as one of the most influential figures in Western and Christian history.

Born in Eisleben, Luther was ordained to the priesthood in 1507. He came to reject several teachings and practices of the contemporary Roman Catholic Church, in particular the view on indulgences and papal authority. Luther initiated an international debate on these in works like his Ninety-five Theses, which he authored in 1517. In 1520 Pope Leo X demanded that Luther renounce all of his writings, and when Luther refused to do so, excommunicated him in January 1521. Later that year, Holy Roman Emperor Charles V condemned Luther as an outlaw at the Diet of Worms. When Luther died in 1546, his excommunication by Leo X was still in effect.

Luther taught that justification is not earned by any human acts or intents or merit; rather, it is received only as the free gift of God's grace through the believer's faith in Jesus Christ. He held that good works were a necessary fruit of living faith, part of the process of sanctification. Luther's theology challenged the authority and office of the pope and bishops by teaching that the Bible is the only source of divinely revealed knowledge on the Gospel, and opposed sacerdotalism by considering all baptised Christians to be a holy priesthood. Those who identify with these, as well as Luther's wider teachings, are called Lutherans, although Luther insisted on Christian or Evangelical (German: evangelisch), as the only acceptable names for individuals who professed Christ.

Luther's translation of the Bible into German made the Bible vastly more accessible to the laity, which had a tremendous impact on both the Church and German culture. It fostered the development of a standard version of the German language, added several principles to the art of translation, and influenced the writing of an English translation, the Tyndale Bible. His hymns influenced the development of singing in Protestant churches. His marriage to Katharina von Bora, a former nun, set a model for the practice of clerical marriage, allowing Protestant clergy to marry.

In two of his later works, such as in On the Jews and Their Lies, Luther expressed staunchly antisemitic views, calling for the expulsion of Jews and the burning of synagogues. These works also targeted Roman Catholics, Anabaptists, and nontrinitarian Christians. Luther did not directly advocate the murder of Jews; however, some historians contend that his rhetoric encouraged antisemitism in Germany and the emergence, centuries later, of the Nazi Party.