Martin Luther's primary focus during the 16th century reforms was to challenge the practices and doctrines of the Catholic Church, particularly its sale of indulgences and the notion of salvation through works. He emphasized "justification by faith," advocating that faith alone, rather than deeds, was sufficient for salvation. Luther's 95 Theses, nailed to the church door in 1517, sparked widespread debate, leading to the Protestant Reformation. His ideas encouraged individuals to read the Bible in their own languages, promoting personal interpretation of scripture and fostering the development of various Protestant denominations.