The main cause of the Protestant Reformation, which began in the early 16th century, was the widespread dissatisfaction with the practices and doctrines of the Roman Catholic Church. Key issues included the sale of indulgences, corruption among the clergy, and the lack of access to the Scriptures in the vernacular. Martin Luther, a German monk, became a central figure in the Reformation with his 95 Theses, which criticized church practices and called for reform. This movement led to a significant religious, cultural, and political transformation in Europe, giving rise to various Protestant denominations and challenging the authority of the Catholic Church.