The greatest leaders of the Reformation undoubtedly were Martin Luther and John Calvin. Martin Luther precipitated the Reformation with his critiques of both the practices and the theology of the Roman Catholic Church. John Calvin was the most important figure in the second generation of the Reformation, and his interpretation of Christianity, known as Calvinism, deeply influenced many areas of Protestant thought. Other figures included Pope Leo X, who excommunicated Luther; the Holy Roman emperor Charles V, who essentially declared war on Protestantism; Henry VIII, king of England, who presided over the establishment of an independent Church of England; and Huldrych Zwingli, a Swiss reformer.