John Calvin was a French theologian and reformer in the 16th century, known for his role in the Protestant Reformation. He emphasized the sovereignty of God and the authority of Scripture, which led to the development of Calvinism, a major branch of Protestantism. His most significant work, the "Institutes of the Christian Religion," laid out his theological ideas and influenced many Christian denominations. Calvin established a theocratic government in Geneva, Switzerland, where he implemented his religious reforms. His teachings on predestination and the importance of a disciplined Christian life shaped the beliefs and practices of many followers. Today, his legacy continues to impact various Protestant communities worldwide.