Martin Luther's challenge refers to the profound opposition he posed to the practices and doctrines of the Catholic Church during the early 16th century, which sparked the Protestant Reformation. His 95 Theses, nailed to the church door in Wittenberg in 1517, criticized the sale of indulgences and emphasized salvation through faith alone, rather than through works. This challenge fundamentally questioned the authority of the Church and advocated for a return to biblical teachings, leading to significant religious, social, and political upheaval across Europe.