The final three books of The Confessions of St. Augustine are themselves a trinity, whose purpose is to better understand the Christian Trinity of the Father, the Son, and the Holy Spirit. As the Trinity is merely three different aspects of the same god, the three books each look at a different aspect while all trying to explain the whole.
The first of the three books concerns itself with understanding the nature of the universe's creation, which is the realm of the Father, who is believed by Augustine to have created all things from nothing. The second book concerns the Holy Spirit, as it's focus is on the different interpretations of Genesis, which was written by Moses who is said to have received the knowledge from the Holy Spirit. The third, and final book of the text speaks of the nature humanity and its place in the universe. As the Son, Jesus Christ is held to be the savior of humanity, this book concerns him.
In Book XI of his Confessions Augustine has finished telling his life story, and now works towards a new purpose, which is to use the philosophies learned from the Neoplatonists to better understand the Bible, and more so, the Trinity itself. He begins by examining the book of Genesis, which speaks of God's creation of the universe. He explores the nature of time itself, and attempts to understand not only what was created, but how it was created and when, and what existed before that. This is an attempt on his part to understand the Father, as the he believes the father to be responsible for the creation of all things, including the Holy Spirit and the Son. This does not prevent the other incarnations from being coeternal with him, however, as they were born of the Father before he created everything else. In understanding the Father, he must first come to an understanding of his creations, with time being the most difficult, as the bible mentions it the least. An understanding of time is needed to understand what the Father was doing before he created the universe, and why it took him an eternity to decide to do so. In the end, Augustine explains that God created time and space simultaneously, so that before the universe's creation, there was no "before" as time did not yet exist, and that time is little more than the order imposed on things by our minds.
The next book concerns itself, not with creation as it happened, but as it was written. Augustine seeks to find which interpretation of Moses writing in Genesis is the truth. He chooses to disregard all those who disagree with the writings themselves, as such people can not be reasoned with, and instead tries to reconcile the arguments of those who disagree with his own interpretations of how creation occurred. If by seeking to understand the work of the Father, he was seeking to understand the Father, than in this way Augustine is seeking to understand the Holy Spirit, as the Spirit is the link between God and man, and was the Father's gift to humanity. In keeping with a unified Trinity however, by understanding the work of the Holy Spirit, Augustine was also improving his understanding of the Father. The part of Genesis he chooses to focus on is the part which concentrates on the very beginning, when everything was create out of nothing. His belief is that first the Father created a formless something that cannot be defined, and it was from this formlessness he created heaven and earth. The other interpretations he looks at include the belief that when Moses said that "In the beginning God made heaven and earth" , he meant that he created the entire visible world, over the course of the six days as mentioned in Genesis. His final conclusion is that there is no one true answer, as "God has tempered the sacred books to the interpretations of many, who could come to see a diversity of truths" . In short, he decides that the bible can mean different things to different people.
The final book of Confessions is devoted to humanity, its creation, and its role in the universe. Augustine states that in the beginning, Adam and Eve were closer to the perfection of God than mankind has ever been since, and as such were all but immortal. It was their fall from grace, and thus the fall of humanity itself, that brought us to the point we are at now. As humanity was created by the Father from nothing, and not from himself, it can never be perfect, as only God is perfect. Such imperfection is displeasing to a perfect God however, and therefore mankind must constantly strive for perfection to please their creator. This is why the final book is the one representing the Son, Jesus Christ. While he was a man in body, Christ was created by God, of God, and in fact was God. It is for this reason that he alone was perfect, and why to attain perfection mankind must follow the teachings of Christ, and to live with him as their example. While mankind can never be perfect as it was not created of God, it also cannot be completely imperfect, as it was created in God's image. It is for this reason that mankind is held to be below god and the Heavens, while still above the rest of gods creatures. For Augustine writes "he received power over the fish of the sea and the birds of heaven and all cattle and all earth and all creeping things which creep on the earth. He judges and approves what is right and disapproves what is wrong" . While humanity certainly is not what it once at the time of creation, we have not yet fallen so far as to lose our status as the favored race of God. It is for this reason that he came among man in the form of the Son, so that he may attempt to redeem us and wash away our sins.
The final book of the text also speaks of the nature of the Trinity, but stops short of defining it, for "Who can understand the omnipotent Trinity?" . Augustine states that it is difficult for the human mind to comprehend the concept of something that is three things at once, while at the same time all of those three are still the same thing. In his mind it is impossible for anything other than a Supreme Being to truly comprehend the nature of a supreme Being. Nevertheless, Augustine puts up a valiant effort to do just that in the final three books of his Confessions. While the descriptions themselves may not do justice to the truly incredible nature of God in his various incarnations, the structure of his attempt somewhat mirrors the point he is trying to get across. Three distinct books, each with the same purpose, which is to discern the nature of God. Each book looks at the same event to help reach an understanding, that is the creation of all things. However, each is distinct in how it goes about finding an answer. One looks at the act of creation in the hopes of understanding the creator. The next looks at the knowledge man received of the creation in the hopes of understanding he who imparts such divine knowledge. The final one looks at the creation and purpose of man himself, in the hopes of understanding the ideal which mankind strives to achieve. Each books argument supports and builds upon the arguments of the other two books, but each in its own way.
In other words, Una trinitas et trina unitas - "One in three and three in one"