In an entry on the filioque, the great historian of doctrine Jaroslav Pelikan wrote,

I am happy to announce that with the publication of Siecienski's book on the filioque, Pelikan's worry about the fate of historians in Dante's inferno has now been substantially laid to rest. The punishment of theologians that might have lasted “several eons of eternity” in that subdivision of hell has been largely, though not entirely, removed. Historians of doctrine will no longer be confined to “the thorough study of all the treatises” devoted to the filioque because Siecienski has done much of that work for us in his book.

This book is the first complete English-language history of the filioque written in the past century. The great strengths of this book are its methodology, analyses of Greek and Latin sources, and clarity and relative brevity of text. First, its methodology spans the history of the filioque from beginning to the present. Most books on the subject focus only on individual authors (for example, Photius) or church councils (for example, Lyons, Ferrara-Florence). This one begins where the solution to all theological controversies must begin and end, namely, the Bible. Siecienski's opening chapter, “The Procession of the Holy Spirit in the New Testament,” explores the biblical foundations of the doctrine with special reference to pneumatology and key filioquist texts, such as John 15:26. This exegetical approach is important especially since so much historical theology is done apart from its scriptural moorings. Within the ongoing life of the church, the scriptures—not the materials of history—remain the source of Christian doctrine. A close reading of the author's footnotes, however, occasionally reveals a misunderstanding of scholars. For example, on page 17 he cites Murray Harris's book, Jesus as God: The New Testament Use of Theos in Reference to Jesus (Grand Rapids, Mich.: Baker Academic, 1998) to support his assertion that “modern exegetes question whether the New Testament ever explicitly refers to Jesus as ‘God’ or whether Jesus thought of himself as such” (17). Having studied with Professor Harris myself, his book supports just the opposite, though in a highly qualified and meticulously nuanced manner. But this should in no way overshadow the larger accomplishments of Siecienski's book.

Siecienski's interpretation of Greek and Latin sources gives readers a penetrating analysis of key patristic and medieval theologians who have shaped the controversy. In an evenhanded manner, the author includes separate chapters on “The Greek Fathers” and “The Latin West.” Until now, in the English-speaking world, most translated sources have been taken from the Greek tradition rather than the Latin one. This has often prejudiced the perspective of readers against the Latin tradition. Now, however, we have in one convenient location the whole story of the controversy as it occurred in both halves of Christendom, East and West. The subsequent chapter titles reveal the balance: “The Filioque from the Seventh to the Eleventh Century”; “The Filioque from the Eleventh to the Thirteenth Century”; “The Council of Lyons to the Eve of Ferrara-Florence”; “The Council of Ferrara-Florence (1438–1439)”; “From Florence to the Modern Era”; and “The Twentieth and Twenty-First Centuries.”

Even though the book is free of confessional bias, the author has made a crucially important discovery in his chapter titled “Maximus the Confessor.” This seventh-century monk and theologian of the Eastern Church is one of the earliest authorities to take an irenic approach to the problem. Siecienski demonstrates how Maximus's Trinitarian theology can “challenge both the traditional Eastern and Western understandings of the legitimacy of the doctrine” (85). The exposition he provides is far more complex than we can summarize here. But the conclusion the author reaches places Maximus right in the center of contemporary ecumenical dialogue between the Catholic and Orthodox Churches. This chapter alone should create the humility needed for genuine dialogue to take place.

Because of the clarity and brevity of its methodology and textual analysis, The Filioque is destined to become a classic on the subject for decades to come.