A. Historical Background on Augustine
Augustine (354-430 CE) was reared in Tagaste, North Africa (modern-day Algeria). He was baptized in 387 CE and eventually became a prominent bishop of the western church. Augustine later died in Hippo Regius (Algeria, North of Tagaste) during a Vandal attack. Aside from Jesus and the Apostle Paul, he evidently had the most profound influence on western ecclesiastical thought. Training in rhetoric could be added to Augustine's résumé. But most germane for our purposes, it is good to observe that the bishop is a divine command theorist, who emphasizes obedience to God: he believes that morality is wholly dependent on the Judeo-Christian God's statutes.
B. Basic Concepts in Augustine's Ethical System
The summum bonum in Augustine is God or eternal life; the divine sphere is humanity's ultimate end. This ethicist/bishop consequently believes that genuine happiness cannot be found in this world--especially not by secular means. He therefore contends that any attempt to master virtue (whether moral or intellectual) is futile: no system of thought divorced from theism can result in genuine happiness or virtuous character. Yet Augustine's ethical system is manifestly informed by Neoplatonism, a philosophy that he develops within the context of an orthodox ecclesiastical framework. The bishop was a Neoplatonist before he converted to Christianity although the extent of his conceptual departure from Neoplatonism has been the subject of scholarly debate at times. Did Augustine convert to Christianity or was Christianity converted to Neoplatonic philosophy by the rhetorical ecclesiastic?
Plotinus (205-270 CE) is usually credited with being the founder of Neoplatonism. He was born in Upper Egypt (Lycopolis) but studied in Alexandria under Ammonius Saccas. Plotinus developed an influential monistic system that posited three "hypostases": the One, Intellect, and Soul. He maintained that all things emanate from and (in due course) return to the One (Ultimate Reality). You can think of this divine movement that originates from the three hypostases as an example of egressus and regressus, a legendary motif that commonly appears in world literature. Hinduism likewise explains the phenomenal world's origin by means of a narrative that accounts for the source of everything plural and diverse through the One (Brahman). Plurality and diversity eventually return to this one unitary ground of being.
Augustine (the converted Neoplatonist) juxtaposes the city of God with the city of man. The city language is metaphorical imagery rooted in New Testament passages from the Revelation of John (e.g., Babylon the Great versus New Jerusalem). Each city represents two classes of humanity. Augustine's metaphorical language also points to the ongoing struggle between fleshly and spiritual people, between unregenerate and regenerate humanity (City of God 14). Those who belong to the city of God cherish divine values as the summum bonum. Conversely, those who align themselves with the city of man love pleasure or desire bodily goods more than goods of the soul. Augustine summarizes the utmost good of humanity in Confessions 1.1: "Our hearts are restless until they repose in thee" (quia fecisti nos ad te et inquietum est cor nostrum, donec requiescat in te). While one may look for God in external things, the idea posited in Augustinian writings is that God (in some sense) actually dwells within us; we just need to acknowledge the supreme presence that abides internally. He avers that our hearts will be troubled until we repose in God, so our eternal peace and very life purpose consequently become associated with the divine will.