Lutheranism is a major branch of Western Christianity that identifies with the theology of Martin Luther, a German reformer. Luther's efforts to reform the theology and practice of the Roman Catholic Church launched the Protestant Reformation. Beginning with the 95 Theses, Luther's writings were disseminated internationally, spreading the ideas of the Reformation beyond the ability of governmental and churchly authorities to control it.

The name "Lutheran" originated as a derogatory term used against Luther by Johann Eck during the Leipzig Debate in July 1519. Eck and other Roman Catholics followed the traditional practice of naming a heresy after its leader, thus labeling all who identified with the theology of Martin Luther as Lutherans. Martin Luther always disliked the term, preferring instead to describe the reform movement with the term "Evangelical", which was derived from euangelion, a Greek word meaning "good news", i.e. "Gospel." Lutherans themselves began to use the term in the middle of the 16th century in order to distinguish themselves from other groups, such as Philippists and Calvinists. In 1597, theologians in Wittenberg used the title "Lutheran" to describe their church.

 Martin Luther (help·info) (10 November 1483 – 18 February 1546) was a German monk, priest, professor of theology and iconic figure of the Protestant Reformation. He strongly disputed the claim that freedom from God's punishment for sin could be purchased with money. He confronted indulgence salesman Johann Tetzel with his Ninety-Five Theses in 1517. 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 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.