The primary causes of the Reformation in Europe during the 16th century included widespread dissatisfaction with the Catholic Church, corruption within the clergy, and the sale of indulgences. Critics like Martin Luther and John Calvin emphasized the need for reform, advocating for a return to the Scriptures and faith as the basis for salvation. Additionally, humanist ideas encouraged questioning traditional authority and promoted individual interpretation of the Bible. Political factors, such as the rise of nation-states and the desire for greater autonomy from papal authority, also fueled the movement. Collectively, these causes spurred a seismic shift in religious, political, and cultural landscapes across Europe.