Final answer:
The title of the book written by John Calvin in 1535 is The Institutes of the Christian Religion.
Step-by-step explanation:
The title of the book written by John Calvin in 1535, which served as a defense of his theology and became the most influential and comprehensive book on Protestant thought during the Reformation, is The Institutes of the Christian Religion. This book outlined Calvin's theological beliefs and had a significant impact on the development of Protestantism.