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 thelogy 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.
Indeed, 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.
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.
(Continued on the next page.)