Final answer:
At the age of 31, Augustine of Hippo experienced a spiritual and philosophical awakening which led him to convert to Christianity, thereafter becoming a central figure in theological and philosophical discourse with works addressing free will, the nature of time, and other critical issues.
Step-by-step explanation:
St. Augustine of Hippo was about 31 years old when he realized that he had made no spiritual commitment and had not settled his spiritual and philosophical issues. This realization and subsequent conversion occurred around 386 CE when he was in Italy, influenced by his readings about St. Antony of the Desert and his experiences teaching rhetoric at the imperial court in Milan.
Augustine's conversion marked a momentous turn, where he started to renounce his previous life of secular ambitions and pleasures, dedicating himself to Christianity and eventually becoming a significant theological and philosophical figure. His writings, like 'Confessions' and 'The City of God,' express his complex thoughts on topics such as free will, the nature of time, and the fall of the Roman Empire in relation to Christian teachings.
Moreover, as Bishop of Hippo, Augustine fought against heresies such as Pelagianism and Manichaeism, establishing definitions of evil, grace, and divine order that would influence Western thought for centuries to come.