When it comes to works of systematic theology—let alone theology in general—I prefer authors that write simply (not simplistically) and succinctly, eschewing dense words and obtuse grammar that signal the author is part of the academic guild and wants the guild to know it. While there is a place for such writing, I prefer reading scholars who write for the benefit of the church. Robert Letham’s “The Eternal Son” (P&R, 2025) does just that. 

I first encountered Letham during seminary, nearly 25 years ago, with his short work, “The Lord’s Supper: Eternal Word in Broken Bread.” That work roots the sacrament in the New Testament, tracing its meaning and interpretation historically across traditions, narrowing its focus to the meaning of the sacrament within the conservative Reformed tradition (highlighting both Calvin’s thought and the Westminster Standards), and then explicating the right administration of the sacrament as an ordinary means of grace within local congregations.

Just as the Lord’s Supper should be a regular feature of any Presbyterian church’s life, Christians should center their life and practice on Jesus Christ, the son of the Living God who has fully revealed his Father and united us to himself through the Holy Spirit. But as Karl Rahner pointed out nearly 20 years ago, American Christians of all stripes hardly know God as triune, let alone understand the persons of the Trinity with any depth. 

Where I live in south-central Alabama, Christians tend to focus on one of the members of the Trinity at the expense of the others: some prefer the Father, others the Son, still others the Spirit, and many land simply on “God.” The Nicene Creed speaks of Jesus as “the only-begotten Son of God, begotten of his Father before all worlds; God of God, Light of Light, very God of very God; begotten, not made, being of one substance with the Father, by whom all things were made. . .”. Yet many well-intentioned, Bible-believing Christians (including within the PCA) have no idea what the Nicene Creed means or why it matters.

Letham writes “The Eternal Son” with the same basic framework as “The Lord’s Supper.” The book presents the biblical foundations of the doctrine, its historical reception and debate, and its theological meaning. Only, “The Eternal Son” (as indicated by its page count) treads much deeper. It assumes and is in conversation with his previous project on the Trinity (2004; revised and expanded, 2019) even as it also engages with his recent work on the Holy Spirit (2023). 

“The Eternal Son” adds to Letham’s already-impressive list of works on Christology: “The Work of Christ” (1993), “Union with Christ” (2011), and “The Message of the Person of Christ: The Word Made Flesh” (2013). Even so, the reader need not have read Letham’s previous works to understand or benefit from his present offering. It is a standalone work that frames the person and work of Jesus Christ in relation to the Father and the Spirit, making it a thorough-going work of Trinitarian theology.  

Letham is unapologetically a Reformed conservative, and to my knowledge, he never leaves the boundaries of the Westminster Standards. Still, one of this book’s selling points is that Letham’s treatment of the subject (and all of its many rabbit trails) is purposefully rooted in Scripture, even as he interacts with theologians and scholars of diverse traditions across Christian history. He is just as apt to exegete biblical texts as he is to converse with Augustine (354–430) or Anselm (1033–1109) or reference T.F. Torrance (20th century).

Some subjects are easier to engage with, while others are more difficult. But it’s not merely that some are difficult; it’s the sheer number of difficult subjects contained within the history of the doctrine. Take, for example, chapter three, “The Word Became Flesh.” Within that chapter alone, Letham takes up the following related and overlapping issues:

• Why the Son became flesh and not the Father or the Spirit
• How the incarnation establishes the compatibility of God and humanity without destroying the Creator/creature distinction
• How the personal identity between the Eternal and the incarnate Son works
• The virgin conception, as in, the conception of the Son of God by the Holy Spirit in the virgin Mary (as opposed to how Catholics have taken the virgin birth and turned Mary into something she is not), and why it matters to highlight the Spirit’s action
• The enfleshment of the Son and what it entails for the Eternal Son to become fully human with two natures 
• The two states of Christ’s humiliation and exaltation
• How God the Son suffers as a man and how that squares with the doctrine of God

Though Letham’s prose is as good as any on offer, because of the nature of the doctrine itself and its attendant questions, sub-questions, debates, and heresies, Letham must get into the weeds. But one of the wonderful aspects of this book is the glossary. Here’s a sample of two different entries:

• Aphthartodocetism: The belief that since Jesus was without blemish or spot, he could not suffer. This would be an effective denial of the genuineness of his humanity, which, being contingent and finite, was inherently liable to constraint by external forces. 
• extra Calvinisticum: The belief of the Reformed church, in agreement with the catholic tradition, that the Son exists beyond the bounds of the human nature assumed into union in the incarnation. The term was coined by the Lutherans, who claimed that the assumed humanity received the divine attributes, including omnipresence, by virtue of the hypostatic union. 

The first term—aphthartodocetism—was new to me. The second term—extra Calvinisiticum—I was very familiar with because it comes up in any good systematic theology coursework. Letham offers a good and helpful definition, but notice that there are other terms in bold print: “nature,” “attributes,” and “hypostatic union.” I suspect that, like a hyperlink on a website, these words are in bold print so that the reader will look them up, too, and see the connection between all the terms in question. 

My point in highlighting two glossary entries is both to demonstrate (again) just how thorough Letham’s work is leaving few, if any, stones unturned, even as he has provided tools to help the reader navigate through the deep waters of the subject. 

And the waters are indeed deep. Is the book difficult? Yes. But difficult in the sense that the subject matter itself can be difficult at points, which is no fault of Letham’s. To that end, it is an excellent resource for any pastor to have on his shelf. For an ambitious Sunday School class or small group with a seasoned teacher or leader, this book could be used for a study. At the very least, it will be very helpful for discussion on the second article of the Nicene Creed. 

For the mature Christian who is already adept at wading into theological works, this book is worth the purchase. It deserves multiple, patient readings. I admit that I left far too much meat on the bone on my first reading and need to return to it again. Even so, my suspicion is that with time, like his work on the Lord’s Supper, “The Eternal Son” will prove to be yet another gem. 

Rob Fossett serves as pastor of First Presbyterian Church in Greenville, Alabama.