Book XIII is the most prayerful of Books in a work that is, in truth, one long philosophical prayer. In this Book Augustine "sums up" the points he has laboriously proven in the previous Books, and also touches on some other points of Catholic doctrine.
First and foremost, it is important to Augustine that everyone remembers that everything a human being does comes from God - even the impulse to have faith in God. All goodness and being come from God, and to think of humanity as separate from God is to misunderstand its nature. All of creation is a function of the goodness of God, he explains, and God made creation out of the abundance of that goodness. The Trinity is touched upon - specifically, the Spirit moving over the waters in Genesis - and Augustine confirms the three-in-one nature (a mystery) of God. Augustine has even more creative linguistic arguments for the rest of Genesis, interpreting much of the narrative as a metaphor for the errors of humanity and the Church. Here began what would become the medieval theological practice of reading a lesson for the earthly Church into all of God's creation and Scriptures. He also affirms the importance and sacredness of the Sabbath.
Augustine muses on his own story of conversion, and how he, like the rest of humanity, has the perverse tendency to turn away from God. His story should be a lesson for all that even if God is desired by a person, he or she can still easily fall into error if, in anything, he or she turns away from God.
Augustine then begins a curious discussion about the "weight" of sexual desire that keeps his soul down among the material things of earth, rather than letting it float upwards to the material realm of God. But in this Book the sexual desire is transmuted into love, and the "weight" of that is like a flame that reaches upward. In this, Augustine is referring to the Biblical story of Pentecost, when the apostles were visited with tongues of fire. The Holy Spirit is represented often as a flame, and this flame of love can bring the soul closer to God.
Augustine reflects again on the temporality of human beings and the eternal nature of God. He ends with several exhortations to God to guide him, and praise for God and all his works.
Augustine is not resting on his laurels in this last Book, though he does seem to think that he has resolved many of the knotty problems presented in Books X-XII. The reference to the Trinity, which would become such a stumbling block for many Christians during the Rennaisance, is not nearly as tortured or fraught with doubt as many other, seemingly easier, tenets of the faith.
The nothingness of humanity without God cannot be stressed too much for Augustine, and the parallel of Augustine's journey to a generalized view of all souls' journey to God is neatly accomplished. The metaphorical lessons he gained from reading Genesis are surprisingly supple and apt, after the tortured explanations for arcane points of doctrine found in earlier Books.
The song of praise for God is important to Augustine, who had to be mindful of writing philosophy solely for intellectual gratification. The question of whether he might have written some sections of this Book with as much regard for philosophy as he did for theology is up for debate. Nevertheless, this is one of the most complete and thorough treatises on Catholic doctrine in existence. It has never been completely disregarded in the Church since its writing, and Augustine is one of the thirty-three Doctors of the Church. Indeed, Confessions is not only one of the most influential Catholic texts, but also one of the philosophical texts most often read up until the twentieth century by Christians of all denominations. Among Catholic philosophers, perhaps only Thomas Aquinas could be considered greater or more complete in his theological explorations. To understand Augustine's works is to understand much of the underpinnings of all Catholic doctrine.
Augustine's final prayers are particularly beautiful (they were noted in their day for the purity and grace of the Latin). He seems to retract his whole work, hearkening back to his wonderment in Book I, saying that such mysteries of God are unknowable by the feeble mind of man. "What man can enable the human mind to understand this? Which angel can interpret it to an angel? What angel can help a human being to grasp it? Only you can be asked, only you can be begged, only on your door can we knock. Yes, indeed, that is how it is received, how it is found, and how the door is opened."