Which reformer is known for challenging the Catholic Church's practices and advancing the Protestant Reformation?

The reformer known for challenging the Catholic Church's practices and significantly advancing the Protestant Reformation is Martin Luther. In 1517, Luther published his "Ninety-Five Theses," which criticized the Church's sale of indulgences, among other practices that he viewed as corrupt and contrary to the teachings of the Bible. His actions sparked a major theological debate and led to a splitting of the Church, ultimately resulting in the establishment of various Protestant denominations.

Luther's focus on the authority of scripture and the doctrine of justification by faith alone became foundational principles of Protestantism. His translation of the Bible into German also made the scriptures more accessible to the lay population, further promoting reformist ideas. Luther’s challenge to the established Church set in motion a series of events that transformed the religious landscape of Europe, making him a central figure in the Protestant Reformation.