This is so because of its topic, as well as for more mundane factors such as its interrupted composition over a period of sixteen years. The theology of the Trinity is a difficult subject to explain or discuss. References to the Trinity in the Bible are few, and their meaning is not always obvious - indeed, they can easily be read as contradictory.
There is no explicit description of the Trinity in the Scriptures at all. The orthodox view of the Trinity (as three persons in one God) is an inferential conclusion from the Bible that took generations to assemble. In light of these limitations, how much clarity about the Trinity is humanly possible? This problem is sufficiently difficult that many people in the time of Augustine opted to classify it solely as a Mystery of Religion that would be understood in the next life but not in this one.
Image (above): A painting of the Trinity with Saints Augustine and Catherine. A work of the Cusco school of art, centred in that Peruvian town during the 16th-18 centuries.
Augustine undertook two tasks in writing On the Trinity. He first wished to combat non-Trinitarian heresy by showing the support for the concept in the Bible and by showing that the various Biblical references were not inherently contradictory. His second motive was to attempt to understand the Trinity more deeply. "On the Trinity" is a long book, the second longest work that Augustine wrote. He toiled on it intermittently, i.e., in between many other writings and projects, for sixteen years. He might not have finished it at all had not the unauthorised publication of the first twelve "books" (chapters) led him to write the final three in order to avoid having the work available only in an incomplete form.
"On the Trinity" begins with a consideration of the references in the Bible to the Trinity, with the aim of reconciling them and explaining them through the supposition of three equal persons in one God. Augustine is at particular pains to maintain the equality of the persons: that the Son is equal to the Father, and the Holy Spirit equal to both. Of particular concern to Augustine are the references to the Son and Holy Spirit being sent, which led others to imply that the Father who sends must be superior to them. This presentation takes up the first eight books. From there Augustine aims to develop some deeper understanding of the nature of the Trinity. His approach is to use the fact that the human race was created in the image of God. Given this, Augustine reasons, there should be some image of the Trinity in every human being. This leads to the consideration of a succession of trinities: the lover, beloved, and love; memory, understanding, and will; the objects of sense, the will to attend to them, and the sense impressions of them; etc. This presentation, which takes up the next four books, is interesting, but often confusing.
It is easy for the reader to see that the trinities Augustine names as present in human beings are not analogues of the divine Trinity, and it can be difficult to understand how Augustine intends to bring this discussion of the trinities in human beings to any relevant conclusion. It is in the last few books, written after the premature publication of the earlier books, that Augustine works to reverse the centrifugal tendencies of his discussion of the trinities in human beings and unify them into a whole. The trinities in human beings are held up not as exact analogues to that in God, but as a ladder, starting with the most carnal and rising towards the most spiritual. Augustine holds that one does not find within ourselves a single Trinity like that of God, but he proposes that we do find a series of them that we can ascend, and in so doing we may approach the divine Trinity and a deeper understanding of God.
The concluding book contains some of Augustine's best theological writing, and at the same time admits that words ultimately fail the task of telling us about God. The great value of De Trinitate lies most in its integration of theological investigation and prayer - of theology and anthropology. It presents our approaching trinitarian theology as we would the Christian task of living in Christ. Trinitarian theology is not approached as a thesis but rather as something that needs to enlighten and enliven our relationship with God - our inclusion within the trinitarian life.