John Calvin (1509–1564) was a French theologian and key figure in the Protestant Reformation. He is best known for his influential work Institutes of the Christian Religion, which systematized Protestant theology. Calvin emphasized God’s sovereignty, predestination, and the authority of Scripture. He established a theocratic government in Geneva, where his teachings shaped the city’s religious and political life. His ideas laid the foundation for Calvinism, influencing Reformed churches across Europe and later in America. Calvin’s legacy remains significant in theology, education, and governance, marking him as one of the most important reformers alongside Martin Luther.