Luther refers to Martin Luther, a German theologian and key figure in the Protestant Reformation, who challenged the teachings and practices of the Roman Catholic Church in the early 16th century. His actions, including the posting of the Ninety-Five Theses in 1517, sparked a movement that led to significant religious, political, and social changes across Europe. Luther's emphasis on faith and scripture as the basis for salvation reshaped Christian thought and contributed to the emergence of various Protestant denominations.