Final answer:
John Calvin wrote the Institutes of the Christian Religion to explain and systematize Protestant beliefs during the Reformation, distinguishing them from Catholic doctrines, especially on the issue of salvation through faith alone.
Step-by-step explanation:
John Calvin wrote the Institutes of the Christian Religion to explain the Reformation and Protestant beliefs. His work provided a systematic theological framework for the new Protestant faith, diverging from Catholic doctrines on issues such as salvation, which he, like Martin Luther, viewed as attainable through faith alone, not through good works or the purchase of indulgences. The Institutes aimed to instruct believers on reformed Christian doctrine, making the case for scriptural authority over church traditions, and became one of the foundational works of Protestantism, affecting various movements across Europe such as Calvinism and influencing the thought of many reformers.