Which of the following is a key figure in the Protestant Reformation?

John Calvin is recognized as a key figure in the Protestant Reformation due to his significant contributions to Reformed theology and his role in the development of the Protestant church in the 16th century. His ideas on predestination, the sovereignty of God, and the authority of Scripture helped shape the beliefs of many Protestant denominations that followed. Calvin's work, particularly his seminal text "Institutes of the Christian Religion," provided a systematic theology that influenced not only the Reformation in Switzerland but also had a lasting impact across Europe and beyond.

In contrast, Thomas Aquinas, while an influential theologian, was part of the medieval Catholic tradition and predated the Reformation. Pope Leo X was a Catholic pope during the Reformation but was known for opposing it and maintaining the Catholic Church's doctrines. Hildegard of Bingen, a 12th-century mystic and writer, contributed significantly to Christian mysticism and music, but her work was not directly connected to the Reformation's theological disputes. Calvin's actions and writings directly addressed and challenged the existing religious structures, marking him as a pivotal player in ushering in significant changes during this transformative period in history.