The key influence on Martin Luther's Ninety-Five Theses was the sale of indulgences by the Catholic Church. In the early 16th century, the Church offered indulgences as a way for people to atone for sins, promising reduced time in purgatory. This practice infuriated Luther, a German monk, who believed it undermined true repentance and contradicted the teachings of the Bible. His 95 Theses, posted in 1517, criticizes indulgences and calls for reform within the church, ultimately sparking the Protestant Reformation and reshaping Christianity in Europe.