The doctrine of the Trinity is one of the high mysteries of the Christian faith. The Christian God is love because he is one God in three Persons. Nothing could be more important than that we have a clear understanding of the Trinity. If we are in error on this point, we do not know God as he really is. The first centuries of Church history are testimony to the fact that this subject is beset with dangers and pitfalls. The Church struggled to find the right language to express the Biblical revelation of the oneness and threeness of God. The result of the struggle is the creedal legacy of the Church. The relationship between the One and the Three in God was clearly defined to enable the faithful to think clearly about, worship and serve their triune Lord. Dangerous and erroneous speculations such as Arianism and Sabellianism were excluded as sub-Christian.
Letham admits that “When it comes to the doctrine of the Trinity, evangelicals have underachieved.” The doctrine of the Trinity is often neglected in evangelical church life. The writer sets out to redress this lacuna. First he examines the Biblical foundations of the Trinity. He gives a helpful and thorough exposition of the Old Testament background to the doctrine. Then he focuses on the New Testament’s portrayal of the relationship between Jesus and the Father. Having established that the New Testament teaches that Jesus is to be worshipped as God alongside the Father, Letham gives attention to the Holy Spirit and triadic patterns. The Spirit is often placed besides the Father and the Son in New Testament formulae, indicating that he is included in the unity in diversity of the Godhead. He concludes,
“The problem of the trinity was being raised and answered in the New Testament. It arose because of Christian experience, worship and thought. It was based upon the life and ministry of Jesus, and his reception of the Holy Spirit, and then upon his resurrection and subsequent impartation of the Spirit to his church.” (p. 71).
Next, Letham traces the historical development of the doctrine of the Trinity. He begins with Irenaeus and Tertullian and then discusses the Arian controversy. The result of that controversy was the Nicaean Creed, which, using the language of Athanasius confessed that God is one being (ousia) and three persons (hypostasis). Athanasius had found language to express accurately the oneness and threeness of God. Christ was confessed as homoousios - of the same essence as the Father
With great historical and theological acumen, Letham discusses the contribution of the Cappadocians, Augustine and John Calvin to the doctrine of the Trinity. One of his favourite theologians is Gregory of Nazianzen, one of whose orations on the Trinity is quoted or alluded to several times in the book. The writer is critical of Western triniratianism from Augustine onwards for so emphasising the oneness of God that the three Persons become problematic. He appreciates the Eastern focus on the three Persons dwelling perichoretically in the one God. But he suggests that the Eastern attempt to distinguish between the energies and essence of God undermines the reliability of divine self-revelation. Letham attempts to mediate in the dispute between East and West over the filioque controversy. His proposal is that the Spirit proceeds from the Father in the Son. Whether this formula will settle nearly a thousand years of theological argument only time will tell.
Letham devotes several fascinating chapters to charting the course of modern discussion of the Trinity. He gives attention to the contributions of Barth, Rahner, Moltman, Pannernberg in the West and Bulgakov, Losky and Staniloae in the East. Letham deals with each writer’s views fairly, analysing the strengths and weaknesses of their approach. An appreciative, but not uncritical chapter is also devoted to T. F. Torrance’s trinitarianism.
The final part of the book concerns critical issues such as the Trinity and the Incarnation, Worship and Prayer, Creation and Mission and the Trinity and Persons. Letham avoids modalism by arguing that only the Son could have become Man. He movingly reminds us that the heart of God is truly revealed in the humiliation and exaltation of the incarnate Son. Our God is one who stoops to serve and die. When thinking of the Trinity, we should not focus on the oneness of God at the expense of the Persons - modalism, or the Persons at the expense of the oneness – tritheism. Both the one divine essence and the three Persons are equally ultimate.
"No sooner do I conceive of the One than I am illumined by the Splendour of the Three; no sooner do I distinguish Them than I am carried back to the One." (Gregory of Nazianzus, here)
Under the heading of mission, Letham discusses the two great challenges to Christianity in the West: Postmodernism with its fragmentary emphasis on diversity without unity and Islam, exemplifying unity without diversity. Only a robustly Trinitarian faith is able to face these challenges to the gospel.
The whole book is shot through with worship, devotion and practical application. Letham has certainly not underachieved. He has helped to think clearly about our Triune God. He calls us to make the Trinity central to mission, worship and prayer. This book is an important contribition to trinitarian theology from an Evangelical and Reformed perspective.