Final answer:
John Calvin is the theologian who introduced the main ideas of Protestantism in his book 'The Institutes of the Christian Religion' and developed the Protestant denomination known as Calvinism.
Step-by-step explanation:
The theologian who set forth the main ideas of Protestantism in his 1535 book "The Institutes of the Christian Religion" was John Calvin. John Calvin was a French lawyer who, after being exiled for his Protestant views, settled in Geneva, Switzerland. There, he developed Calvinism, a significant Protestant denomination that emerged following Lutheranism. Calvin's influence on Protestantism includes his advocacy for making the Bible accessible to ordinary people, asserting that believers could understand the power of God through scripture and reflection on their spiritual condition.