The primary aim of Martin Luther's 95 Theses was to challenge the practices and beliefs of the Roman Catholic Church, specifically its sale of indulgences, which allowed people to pay for the remission of their sins. Posted in 1517, Luther's theses questioned the authority of the Church and argued for salvation through faith alone, rather than through works or payments. This document sparked the Protestant Reformation, leading to the establishment of various Protestant denominations and significant changes within Christianity. Luther’s actions encouraged religious debate and ultimately transformed European society.