The primary catalyst for the Protestant Reformation in Germany was the practice of selling indulgences by the Catholic Church. Indulgences were certificates issued by the church that promised a reduction in the punishment for sins either in purgatory or on Earth.
Johann Tetzel, a Dominican friar and papal commissioner, was particularly aggressive in selling indulgences in Germany in 1517. Tetzel claimed that indulgences could not only forgive past sins but also guarantee salvation. This practice sparked outrage among many people, including Martin Luther, a professor of theology at the University of Wittenberg.
In October 1517, Luther published his Ninety-Five Theses, a list of arguments against the sale of indulgences. He argued that salvation could not be bought through indulgences but rather only through faith in God. The printing press then disseminated the Ninety-Five Theses throughout Germany, sparking a widespread theological debate that marked the onset of the Protestant Reformation.