Augustine wrote The Trinity over twenty years, ending with a text so enormous that it was published in parts, in rough form, and later revised by Augustine (Harmless, 2010, p. 286). It is somewhat unique in Augustine’s major works in that he pursued it, not in response to heresies, but out of his own interest and conviction of the importance of the doctrine. At a high level, the book may be viewed as having two halves – the first half contains Augustine’s theological explanations of the trinity, while the second half discusses how that image is seen in the imago Dei in man. At a more granular level, this 15-book collection can be divided into the four sections discussed below.
Books 1-4: The Trinity in the Creeds and Scriptures
Augustine establishes the doctrine, not first through scripture, but through the creeds and church councils that had already authoritatively clarified the doctrine of the trinity. The Nicene creed began to resolve the Arian controversy, concluding that Jesus was co-equal with the Father. However, anti-Niceans and Arians continued the debate, which was again addressed in the Council at Constantinople in 381 (Harmless, 2010, p. 276). Augustine then moved on to review the core New Testament passages such as in John 1 that attest to the divinity of Jesus and the Holy Spirit. He also traces the many Old Testament epiphanies and passages that allude to the members of the Trinity, and how these fit in and reveal the redemptive mission of God to come in the New Testament, as revealed in God the Father, Son, and Spirit. Augustine also addresses objections and adds qualifications that keep us from various heretical extremes. Many of these distinctions are related to contemporary heresies that Augustine dealt with in his other writings.
Books 5-7: The Trinity and the Names of God
In these books, Augustine advances his arguments into ideas of form and substance, God’s immateriality, and his eternality, which includes Christ’s eternality. He proceeds to examine the names of God, including Lord and Creator, begotten vs. unbegotten, as well as the meanings of Father, Son, and Spirit, and what these titles and relationships mean. The sum of the three, for instance, is not greater than the Father (Levering, 2013, p. 176). Importantly, he defines a difference between the person and essence of God, from which we get the definition of “three persons, one essence.” Lastly, Augustine sees his arguments as an ascent towards knowing God and proposes the primary assumption that the imago Dei demands a reflection of the Trinity in the nature of man.
Books 8-14: The Imago Dei in Human Psychology
In books 8-14, Augustine explores his contention that the trinitarian nature of God is reflected in man, specifically the mind or soul of man. He proposes many triads, though he is not unskeptical, and often points up the possible weaknesses of each. Based on the idea that God is love (1 John 4:7), he first explores the triad of the lover, the beloved, and love itself (Augustine from The Trinity 8:10:14, quoted in Harmless, 2010, p. 297). In book 8, he also introduces us to the paradox of inquiry. That is, how can we discover what we do not know? That is, we don’t know what we don’t know, we don’t even know what we are specifically looking for. The epistemological difficulties of knowing the invisible God, and even what phenomenon or ideas we should be seeking, and by what methods are a significant paradox.
Augustine goes on to elaborate at least two more major triads based on the mind of man, as well as others each based in one of the five senses, in the bodily triad of father, mother, and child, as well as other human capabilities. These two major triads are mind, knowledge, and love, as well as memory, intellect, and will. In all of these, however, he is not merely conjecturing random triads, but is evaluating them according to rules to see if they measure up to a Trinitarian-like triad:
[Augustine] is carefully testing analogies using very specific criteria…at the heart of the doctrine of the Trinity. He is testing whether [the elments]…are (i) the same substance, (ii) equal to each of the others, (iii) distinct (but not different) from one another, (iv) mutually inter-related, (v) mutually indwelling, and (vi) united (but without any mixture) (Harmless, 2010, p. 298)
Book 15: Human Weakness and the Trinitarian Imago Dei
In this final book, Augustine explores some weaknesses in this trinitarian approach to anthropology. He does not believe that such human triads prove the divine Trinity, and he admits that there is no obvious direct analogy. Augustine hides this lack of congruity behind Paul’s admission of incomplete knowledge (“now we see as through a glass darkly, 1 Cor. 13:12), but it may just be that the congruity is a bit of a fabrication, and that our imago Dei has little to do with the Trinity. Augustine provides a helpful review of each of the previous chapters, and reminds us, as he has throughout, that true knowledge of God comes from devotion and interaction with God, not merely intellectual musing.
“Why search for something when you have comprehended the incomprehensibility of what you are searching for? Only this: because you should not give up the search as long as you are making progress in the search itself for things incomprehensible, and because you are becoming better and better by searching for so great a good, which is both search for in order to be found and is found in order to be searched for. For it is both searched for in order that it may be found sweeter, and found in order that it may be searched for all the more eagerly.” (Trinity 15:2.2 in Harmless, 2010, p. 314)
Harmless, W. (2010). Augustine in His Own Words. Catholic University of America Press.
Hoekema, A. A. (1994). Created in God’s Image (Reprint edition). Eerdmans.
Levering, M. (2013). Theology of Augustine: An Introductory Guide To His Most Important Works. Baker Academic.
 It is likely that Augustine focused on the mind, or soul of man because it was in the intelligence and other features of the mind in which the imago dei was supposed to dwell, making us different from the animals. Later theologians moved more towards the “spiritual” function of communion with God and moral awareness, since these seem more distinct rather than possibly a mere matter of degrees (e.g. animals have intelleigence, ours is not different just greater). “Early in the history of Christian theology…man’s intellectual and rational powers were singled out s one of the most important, if not the most important, features of the image of God….Certainly include in the image here is man’s moral sensitivity…and his conscience. Included also is the capacity for religious worship (what Calvin called the sensus divinitatis or “awareness of divinity”). An important human quality frequently mentioned by recent theologians is that of responsibility: man’s ability to respond to God and his fellowmen.” (Hoekema, 1994, p. 70)