What was Martin Luther's area of expertise?

Martin Luther was a German theologian and religious reformer, best known for his role in initiating the Protestant Reformation in the 16th century. His expertise lay in theology, particularly in his critiques of the Catholic Church's practices and doctrines, such as the sale of indulgences. Luther's writings, including the Ninety-Five Theses, emphasized salvation by faith alone and the authority of Scripture, which significantly influenced Christian thought and Western history.