The significant event that Martin Luther sparked in 1517 by posting his 95 Theses was the Protestant Reformation. His theses critiqued the practices of the Catholic Church, particularly the sale of indulgences, and called for reforms within Christianity. By challenging the authority of the Church, Luther initiated a religious movement that ultimately led to the creation of various Protestant denominations. This reformation had profound effects on European society, politics, and religion, contributing to the decline of papal authority and the rise of individual interpretation of the Scriptures.