What was the main difference in doctrinal beliefs between Martin Luther and John
Calvin?

Calvin had the idea that there was two group of people; the Elect and the non-Elect, which were chosen by God to be saved or not. Predestination was a key idea in Calvin's theology. He believed everything was already planned before even you were born. Luther believed that the state and the church should be separated.

Explanation:

Luther believed that the state and the church should be separated. The state should have all the power. While on the other hand Calvin believed that the state and the church should not be subject to one another and vice versa.

Explanation:

i can't beat you to a pulp if i'm not close enough.