Confessions, the most famous work of Saint Augustine, the Bishop of Hippo, is a rather peculiar text of philosophy, both compared to the previous and the following ones. In this great example of rhetorical and philosophical art, there is a long dialogue with three characters: the narrating Augustine, the narrated Augustine and God, who through biblical quotations seems to dialogue and actively partake in the path of thought.
In the thirteen books that make up Confessions, the ‘spatial’ collocation also plays a decisive role. In the first nine books we find ourselves in the port of Ostia, where an Augustine, who had not yet converted to Catholicism, is dealing with classical studies and with his convinced adherence to Manichaeism. In the two following books the ‘space’ is the inner space of memory, of memory and therefore of soul and spirit. The last two books, on the other hand, with their exegesis of the first chapters of Genesis, are placed in the perspective of the Universal, and therefore what allows the full unfolding of Augustinian ontology theory.
In the order of the cosmos, therefore, everything is good according to its own nature and degree.
Saint Augustine is substantially a Neoplatonist, even considering that in the Milan of the 4th-5th century A.D. there are no original texts by either Plato or Aristotle, but only summaries, commentaries and interpretations probably of a clear Platoine/Porfirian style. However, this Platonic inclination of his is clear and manifests itself above all in the way he deals with the theme of creatio ex nihilo. In fact, he is aware of the logical aporia that one runs into when one wants to argue that there is an eternal and creative entity, which not only creates from nothing (and what nothing, since something eternal is present?) but it is not modified in any way by creation itself. The question is: how is it possible that the finite, the becoming, appears alongside the infinite, the eternal, without there being any substantial modification of the former?
Let us deal with this question in its relationship with the thorny theological question of theodicy, that is, the answer to the question “unde malum?” ‒ “where does evil come from?”. As you may recall, in Plotinus eternal matter is the cause of evil, since the perfect formality emanating from the One is necessary to unite with matter, eternal but imperfect, indeterminate, and substantially inadequate for the purpose. Augustine cannot accept this position, since matter too is created by God, and therefore one should admit that divine perfection itself is the creator of the imperfect, and therefore of evil. Augustinian’s observation in this sense will be fundamental for all theology until today: God is supreme Good and supreme Being and creates matter which in turn is Being and therefore Good.
Therefore, within a hierarchical system such as the Cosmos (and every finite and becoming reality is necessarily structured in a certain ordered hierarchy), different degrees of Being, and therefore different degrees of Good, manifest themselves. In other words, if the Being is in identity with the Good, then it is evident that corruptible (‘becoming’) things are a Being (Good) that loses its Being (Good) until annihilation. Becoming, the transition from Non-Being to Being and vice versa, is therefore the shift from Evil to Good and vice versa. So what is evil? It is the same as Nothingness, that is, deprivation, corruption, deterioration of the Being (Good).
Evil is the deprivation of Good – privation boni ‒ just as Nothing is the deprivation of Being. This is how to reconcile the problem of ontology’s multiplicity, of becoming and evil while remaining in the perspective of Christian revelation.
In the order of the cosmos, therefore, everything is good according to its own nature and degree. In fact, Saint Augustine points out that a cosmos containing only the things that have more reality (the good things) would be deprived of those that have less reality, and this would make it worse in itself because it would lack all the being that belongs to ‘inferior’ things. Therefore, the existence of the ‘evil man’ is also good as long as he is placed ‒ as it happens ‒ on the level of ‘inferior’ things. But what is wickedness? Its origin is not in God, but in ‘free will’, that is, in the possibility of choosing what one might reject and vice versa. Sin, therefore, is an error (deviation) of the will, and involves a deprivation of good and a corruption of the soul that loses both being and reality ‒ in the same way that the body is deprived of the soul when it dies. Therefore, to be truly ‘free’ implies being free from the risk of falling into error and being deprived of reality. But this is only possible through divine intercession, that is, through ‘Grace‘. The ‘evils’ that man suffers are also ‘goods’, because their happening is the fruit of ‘divine justice’, and from God’s justice, which is the sum of goodness, can good only come from. In this way Saint Augustine overcomes the Manicheism to which he had adhered in his youth.
Saint Augustine’s position does not totally solve the ontological problem of creatio ex nihilo ‒ and in all his writings there is a certain embarrassment for the author on this particular issue; however, it is a solid foundation on which later theologians will build the great theoretical framework of Christian theology.
 All the quotes from Confessions by Augustine of Hippo are translated by Albert C. Outler, Philadelphia, Westminster Press  (Library of Christian Classics, v. 7).