What was Calvin's major contribution while he was in his office?

John Calvin's major contribution during his tenure in Geneva was the establishment of a systematic and influential framework for Reformed theology. He emphasized the sovereignty of God, the authority of Scripture, and the doctrine of predestination, which shaped Protestant thought. His seminal work, "Institutes of the Christian Religion," provided a comprehensive guide to his theological views and helped to solidify Calvinism as a major branch of Protestantism. Additionally, he implemented a model of church governance that influenced the structure of Reformed churches worldwide.