I heard that St. Augustine of Hippo did put Neoplatonic thoughts in his works like "Confessions" ( I have the book but didn't read it yet).
Is it necessary to read neoplatonic books like the Enneads or others in order to understand the books of St. Augustine ?
How deeply do you want to understand Augustine? You don't need Plotinus to understand the Confessions, but Augustine is vastly more than the Confessions. He's the pivot point between Classical and Christian civilization and his thought dominated the west for the next 1000 years. If you really want to understand him and Christian theology, then you'd better read some Plotinus.
But start with Peter Brown's biography, Augustine of Hippo. He does a wonderful job of placing Augustine in historical and intellectual context. So you can safely use his book to guide further reading.
Or to put it this way: was neoplatonism a critical, not to say essential, element in Augustine's intellectual growth? Unequivocally, yes.
Do you need to study neoplatonism separately in order to understand and appreciate Augustine? Not unless you want to study his more arcane theological works. Such a study will help you understand how he got to where he ended up, but in the end he was not a neoplatonist. He was a Christian and not coincidentally he defined just what that means. He was one of the most fascinating figures in history.
Given that Christianity borrowed a lot from neoplatonism (works of Iamblichus of Chalcis: Theurgy of the Soul, Egyptian Mysteries, Letters), and openly harvested other traditions as a religion that was conjured at the Council of Nicea for the purpose of amassing political power (letters of Constatine I to the Bishops), reading any works antedating it is promoting understanding of later developments. The hierarchies were stripped from Proclus's works (Wilhelm Schmidt-Biggerman), the indivisibility of the Spirit (Monad) from Pythagoreans, the later term is Monoeides (Wilhelm Schmidt Biggerman)
Reading neoplatonic philosophers, which I consider superior to the Doctors of the Church (should I mention a few?) I found it has more to do with Theurgic work then anything Christianity could invent (Theion Ergon: work of humans work of Gods: Personal engagement, instead of a relegated one). Neoplatonism was wrapped around the artificial religion of Christianity forcefully to justify the latter's philosophical foundations. As such Christianity did not exist before the Council of Nicea, there were gnostic sects of various outlooks, some were following Christos (a term from Judaica, soteriology, as understood in Judeo-Christianity, does not exist elsewhere), others were not.
Reading fragments of neoplatonic corpus would be conducive to understand the dynamics of later throught, inclusive of Christian theology.
Given that St. Augistine was a converted pagan of a rather low mind (comparing to other works from his time, I might need to browse my resources to name a few) and for his times he was not the avantgarde of intellect, it would be quite useful to study previous works, or at least track the build-up of his mind to establish how he established what he thought.
One cannot read everything nor does reading something mean that one understands what one has read.
To make sure the reading that one does involves the best understanding it is useful to focus on introductions of these peripheral works, focusing on the selections of the originals referenced by the authors of these introductions.
In the case of the study of Augustine, Plotinus’s Enneads would be a peripheral work. Finding a contemporary introduction or commentary on the Enneads would be preferable to reading the Enneads without such guidance and risking misunderstanding of what one has read.
One work on Plotinus that might be worth considering is Dominic J. O’Meara’s Plotinus: An Introduction to the Enneads, Clarendon Press, Oxford 1995. His perspective attempts to make sense out of the Enneads based on a tradition encompassing Plato, Aristotle, the Stoics, the Epicureans, Augustine, Descartes and others. See his Epilogue for more information on the influence Plotinus had.
The book is short, only 119 pages, but it may require multiple readings. References to the Enneads can be read using the translation by Stephen MacKenna and B. S. Page available online at Sacred Texts.
According to O’Meara (p 113) Augustine's reading of Plotinus “opened the way for his conversion to Christianity and had at first a major impact on his thought. When he became a bishop and a very powerful Church leader, he sought to restrict more and more this Plotinian influence.” One way to approach Augustine would be to keep a question open in one's mind of why Augustine changed his mind about Neo-Platonism.
