In Basel, during his time in exile, John Calvin began working on what would become his most influential theological work, The Institutes of the Christian Religion. This monumental text is widely considered to be one of the most comprehensive and systematic presentations of Protestant theology ever written. Calvin’s ambition with The Institutes was to present a coherent and structured account of Christian doctrine that would serve as a guide for the growing Protestant movement, uniting the fragmented theological ideas that were emerging during the Reformation.

At this stage in his life, Calvin was heavily influenced by the early Reformers, Martin Luther and Huldrych Zwingli. Both of these figures were central to the Protestant Reformation, having challenged Catholic doctrines and practices, particularly in areas such as salvation, the authority of the Pope, and the nature of the Eucharist. Calvin, like Luther and Zwingli, rejected many Catholic teachings, particularly the Church’s stance on salvation and its emphasis on ritual and tradition over Scripture. However, while he respected the theological contributions of Luther and Zwingli, Calvin felt that their approaches lacked the clarity, cohesion, and systematic structure necessary to bring about lasting reform.

Luther, the German theologian who sparked the Reformation with his 95 Theses in 1517, had emphasized justification by faith alone and the authority of Scripture, but his works, though revolutionary, were often not presented in a fully systematic way. Zwingli, a Swiss reformer, also rejected Catholic traditions but focused more on the symbolic nature of the Eucharist and the necessity of reforming church practices according to Scripture. Although both Luther and Zwingli had laid the groundwork for Protestant thought, Calvin believed their teachings, though powerful and foundational, lacked a comprehensive framework that could hold the diverse ideas of the Reformation together.

This gap in clarity and cohesion was what motivated Calvin to write The Institutes. He sought to bring together the fragmented elements of Protestant theology into a single, cohesive framework. His approach was to present theological ideas in a structured, logical manner, addressing topics such as the nature of God, salvation, the sacraments, and church governance. In doing so, Calvin not only built on the foundational theology of Luther and Zwingli but also contributed significantly to the development of a distinctive Reformed tradition that would set Protestantism apart from both Catholicism and the Lutheran branch of the Reformation.

The Institutes was not just an academic exercise for Calvin; it was a pastoral and evangelical work, written with the intention of instructing and guiding believers in their faith. Calvin's systematic approach sought to provide clarity on key theological questions that were dividing Christians at the time, offering a unified vision of Christian doctrine rooted in Scripture. As such, The Institutes became one of the central texts of Reformed Protestantism, influencing not only the theological debates of the Reformation but also shaping the way future generations of Protestants would understand the faith. Calvin's work in Basel ultimately laid the intellectual foundation for the spread of Reformed theology across Europe and, eventually, the world.