Final answer:
John Calvin was most influenced by Luther's idea of justification by faith, which states that salvation is attained through faith alone, not through human actions or indulgences.
Step-by-step explanation:
The idea of Martin Luther's that most influenced John Calvin was the doctrine of justification by faith. Luther believed that it was not through human actions or the purchasing of indulgences but through faith alone that one could attain salvation, an idea that resonated with Calvin. Calvin extended this concept to include predestination, asserting that God's grace could not be influenced by human actions as God had already chosen the 'elect' who were predestined to be saved.