Which German theologian is known for initiating the Protestant Reformation in the 16th century?

The German theologian known for initiating the Protestant Reformation in the 16th century is Martin Luther. His actions, particularly the posting of the Ninety-Five Theses in 1517, directly challenged the practices and doctrines of the Roman Catholic Church, particularly the selling of indulgences. Luther's emphasis on justification by faith alone, the authority of scripture, and the priesthood of all believers fundamentally altered the Christian landscape and led to the establishment of various reformative movements across Europe.

This Reformation movement not only sparked significant religious changes but also had profound social, political, and cultural implications, laying the groundwork for modernity in Europe. Luther's influence extended beyond Germany, inspiring reformers such as John Calvin and Ulrich Zwingli, but he is typically recognized as the pivotal figure who started this significant movement.