Final answer:
St. Augustine of Hippo authored the City of God and greatly influenced early Christian theology by distinguishing between the heavenly city and the flawed earthly city, setting the stage for future discussions on Christian doctrine. The correct option is A.
Step-by-step explanation:
The City of God was written by St. Augustine of Hippo, who significantly influenced the early development of Christian theology. Augustine was not only a prominent theologian of his time but also a philosopher whose works continued to shape medieval European philosophy. He tackled various heresies and reinforced the concept that the Kingdom of God, composed of people who embraced the Christian faith, is eternal, unlike earthly kingdoms which could fall.
As Bishop of Hippo, Augustine produced hundreds of works on Christian doctrine. His profound insights into the nature of time temporality, agency, and free will are still discussed by philosophers today. The role he played in combatting the Donatist and Pelagian heresies further cemented his legacy in Christian history.
Hence, Option A is correct.