The 95 Theses were a series of propositions written by Martin Luther in 1517 that criticized the Catholic Church's practices, particularly the sale of indulgences. This document is often considered the catalyst for the Protestant Reformation, as it sparked widespread debate and dissent against the Church's authority and practices. The 95 Theses highlighted issues of corruption and questioned the Church's teachings, ultimately leading to significant religious, political, and social changes across Europe.