Jackson Lashier argues that previous scholarship has largely done a disservice to Irenaeus of Lyons as a Trinitarian theologian. Most scholars, while acknowledging that Irenaeus’ accounts of God's operation in the economy are Triune, posit that he rejected as speculative any account of God divorced from Scripture. And given that the development of the doctrine of the Trinity is understood to be extra-biblical from the standpoint of biblical higher criticism, any attempt to search for Trinitarian theology in Irenaeus' writings is misguided and anachronistic. Lashier refers to this majority perspective as the first trajectory of Irenaean scholarship. On the other side of the spectrum are found a minority of scholars who are governed by ideological considerations of wanting to find clear lines of doctrinal Trinitarian orthodoxy extending from the New Testament through to the fourth century and beyond, and who therefore read into Irenaeus later Nicene theology; Lashier refers to this perspective as the second trajectory of Irenaean scholarship. To avoid the presumption against a developed Trinitarian theology, characteristic of the first trajectory, as well as to avoid the anachronisms of the second trajectory, Lashier proposes that Irenaeus' Trinitarian theology needs to be studied contextually, by which he means that it needs to be studied in a way that takes full account of his interactions with various Gnostic schools and, even more important, his engagement with the theology of the Apologists. For it is when Irenaeus' Trinitarian theology is contrasted with that of the Apologists that we appreciate the degree to which Irenaeus advanced Trinitarian thought in fourth-century directions.

Lashier's first chapter focuses, therefore, on the dissimilar contexts out of which the Apologists and Irenaeus wrote, arguing that, while Irenaeus certainly knew and was influenced by the work of Apologists like Justin and Theophilus, his interactions with his Gnostic interlocutors compelled him to develop a Trinitarian theology that went beyond the Apologists. This was because Apologetical theology converged in many ways with Gnostic theology, particularly on issues bearing on Trinitarian theology – the divine nature, God's relationship to other divine beings and God's relationship to the world – a convergence that scholarship has not, according to Lashier, hitherto sufficiently acknowledged.

In the chapters that follow, Lashier examines Irenaeus' understanding of God the Father, of the Logos/Son, and of the Sophia/Spirit before concluding with a portrayal of the work of all three in the economy. What emerges from this account is a Trinitarian theology that prefigures Nicene Trinitarian theology. Whereas the Apologists spoke of three divine entities in accordance with the regula fidei, the Logos and Spirit emerge as divine beings ontologically inferior to the Father who is identified as the Creator God of the Hebrew Scriptures. Irenaeus, whom Lashier understands to be governed more by Scripture and the traditional definition of God as spirit than the philosophically-inclined Apologists, develops a Trinitarian theology in the full sense of the word.

Lashier means by this that Irenaeus conceives of three equally divine beings who share in one spiritual nature and who are eternally distinct. To demonstrate this argument, Lashier examines how Irenaeus describes the relationship of the Logos/Son and of the Sophia/Spirit with the Father apart from their economic operations. Irenaeus' emphasis on a simple divine nature translates for him into the understanding that the Father, Logos/Son and Sophia/Spirit must indwell one another completely in an eternal relational unity; Lashier points out that, at first, Irenaeus does not include the Spirit in this interpenetrating relationship, but that he incorporates the Spirit in the later books of Against heresies. This relationship, however, does not make them indistinguishable, and Lashier submits that Irenaeus argues for their eternal distinction through appeal to the generations of the Logos/Son and Sophia/Spirit from the Father as well as through appeal to the economic operations distinct to each. While Irenaeus is reluctant to delve deeply into the generations of the Logos/Son and Sophia/Spirit, his understanding of the generations is governed by an emphasis on the spiritual unity of the Father, Logos/Son and Sophia/Spirit and the simplicity of the divine nature. Thus, Irenaeus removes any time element from these generations and argues that their generations from the Father do not mean that they are separated from him or come out of him; their generations do not compromise their spiritual unity. But their economic operations demonstrate real distinction between Father, Logos/Son and Sophia/Spirit, albeit a distinction that always assumes a prior ontological unity. Lashier argues that the logic of Irenaeus' theology translates into the conclusion that the economic operations of the Logos/Son and Sophia/Spirit necessitate a divine status of ontological equality with the Father.

By assessing Irenaeus' thought in relation to the Apologists, Lashier demonstrates how Irenaeus departs significantly from his theological forebears, and so illustrates how he was, in the face of the Valentinian challenge, forced to develop a Trinitarian logic that pointed towards Nicene theology. Lashier convincingly demonstrates, therefore, the shortcomings of the first trajectory of scholarship noted above that rejected searches for a Trinitarian theology in Irenaeus as anachronistic. That said, there are times in the book when Lashier verges on the precipice of himself anachronistically turning Irenaeus into a Nicene thinker, this despite his stated determination to avoid doing so. On the last page of the book, Lashier helpfully touches on how Irenaeus' Trinitarian theology diverges from Nicene Trinitarian theology, but a more sustained treatment would have helped not only to underline the pitfalls of the second trajectory that Lashier rejects, but also to illustrate more clearly how, despite the divergences between Irenaeus and Nicene Trinitarian theology, Irenaeus' insights influenced Nicene thinkers. This shortcoming aside, Lashier's Irenaeus on the Trinity is, on the whole, a carefully argued monograph that provides a valuable contribution to our understanding of Irenaeus of Lyons and the development of Trinitarian theology prior to Nicaea.