Which of the following was one of Saint Augustine's key ideas?

a. His belief in monastic celibacy

b. His doctrine of Christ's divinity

c. His doctrine of original sin

d. His belief that the pope was Peter's successor

The answer that was one of Saint Augustine's key ideas was C. his doctrine of original sin.

He claimed that the original sin comes from the fall of man - when Eve committed the crime of tasting the apple, which led to her and Adam being expelled from Eden.