Final answer:
St. Augustine of Hippo was an influential figure in late antiquity who addressed the challenges faced by Roman Christians during the decline of the Roman Empire. His work 'The City of God' explained the decline of the Empire as irrelevant to the greater mission of salvation, and he also contributed to the understanding of free will and the conflict between good and sinfulness.
Step-by-step explanation:
St. Augustine of Hippo, a prominent figure in late antiquity, played a crucial role in the intersection of Christian and classical learning. He addressed the challenges faced by Roman Christians during the decline of the Roman Empire. In his work 'The City of God,' Augustine explained the decline of the Empire as irrelevant to the greater mission of salvation. His ideas on free will and the conflict between good and sinfulness also had a significant impact. Additionally, Augustine defended theological orthodoxy against heresies and left an indelible mark on the early church.