Letham's "The Holy Trinity" is an in-depth discussion of this distinguishing, cardinal, yet too-neglected doctrine of Christianity.
Letham's main concern, and crowning section, has to do with the importance of the Trinity. What are its implications for our worship and for our lives in general? As Christians we are saved from sin and death by all means, but we are not only saved "from": we are also saved TO union with God. Letham argues that this attribute of God in particular we should understand better: what is the union within God himself that we are adopted into? How do we relate to the Father, the Son, and Holy Spirit?
Letham charges that too often the church (in particular the Western church) is ignorant of, or negligent of, the Trinity. We refer to God as "LORD" without thinking about who God is or who Father, Son, and Holy Spirit are. We have our analogies handy -- but unfortunately these usually introduce heresy themselves. Even in the best use these are limited to "illustrating" how God is simulataneously One and Three; and we remain ignorant about the relationship among Father, Son, and Holy Spirit, and about how God in each of his persons relates to us (what is the same of the One God, what is unique among each of the three Holy Persons).
As a sneak preview, let me share a few salient points of this doctrine:
* God is knowable to us because he revealed himself to us. And he revealed himself as he truly is, not as a false façade; so while we finite mortals can not know everything about Him, we can trust that what He revealed is true of his nature.
* God is One: one essence, one being.
* God is three persons: God the Father, God the Son, and God the Holy Spirit are each fully God. They are inseparable; each is fully God and fully indwells each other, as compared to being a collection of "1/3 of god's" that add up to a whole.
* God relates to us consistently in a pattern "from the Father, through the Son, by the Holy Spirit". E.g., Compare a common perception of salvation "God the Father is angry at us as sinners, but Jesus died on the cross to save us from sin and all that anger and we're saved when we make a decision for Christ" to its Trinitarian expression in Titus 3:4 - 6: where we are saved because of the love of God [the Father], through the atoning death of Jesus Christ, by the washing and renewal of the Holy Spirit. Too often we misrepresent the Father's role, and ignore the Holy Spirit's role while overplaying our own.
The book is structured around the chronological development of the doctrine: with sections considering:
* Old Testament foundations (where the Trinity is never made explicit, but is strongly alluded to -- from a Christian interpretation; many of my Jewish friends will disagree)
* New Testament foundations, where several authors develop the components of the doctrine of the Trinity
* The early Fathers, who systematized the doctrine and created the term "Trinity" to describe it
* The history of the doctrine within the Church and the many related heresies that arose, and were resolved
* Overview of major current contributions to the doctrine of the Trinity
* Implications of the doctrine of the Trinity for world view, worship, prayer, and missions
Letham generally sticks to serious Christianity rather than it's more mindless offshoots: e.g., he addresses Arianism deeply at the time when the Church dealt with it, but doesn't deal with "Jehovah's Witnesses" who ignore the outcome and preach a doctrine that has long been demonstrated to be false and rejected by the Church.
In a sense this book is written as part of a wider dialog. Rather than being a completely independent work, it is obvious that at times Letham is answering writings by other authors. The result is very readable, and includes a wide range of interlocutors from the breadth of serious Christianity: from the fathers to modern theologians from Eastern Orthodox, Roman Catholic, and Protestant churches. This was rather refreshing in ways: when I run into Rowan Williams' name, usually it has to do with the politics of keeping the Anglican communion together; here here he is dealing with doctrinal issues that the church is about. As a reader, you will be caught up in the dialog developing this theology, rather than watching it passively from your armchair. As befits a complex topic, this is no light reading material. This is written at an academic level, so don't expect to read this casually or quickly; but in reading it, as a Christian, do expect: to be engaged; perhaps to be baffled with how, at times, we fight over things we needn't; but mostly to be rewarded with a deeper knowledge and awe of the God who is there.