This passionately argued book develops the insight that the most potent critiques of religious dogma have been fueled by deep moral conviction. Vocal unbelief sprang not from detached skepticism about the supernatural, contends Dominic Erdozain, but from “a visceral sense of right and wrong” (5). Paradoxically, this sense, which Erdozain dubs “conscience,” was itself rooted in Christian belief. Whether conceived as the divine spark in the soul, inner light, reason, or simply raging indignation, Christian conscience proved “the enduring solvent of orthodoxy” (116). By creating ecclesiastical regimes and dogmatic systems that threatened to diminish dissenters, while also supplying the moral ground from which to attack them, European Christianity thus bred its own critique. Secularization was an intra-religious affair.

“Unbelief” is for Erdozain a broad category, basically synonymous with the rejection of theological orthodoxy. It includes agnosticism and atheism, but many of the “unbelievers” encountered here turn out to be heterodox Christians. That is a key part of the argument: one of Erdozain's chief contributions is to redescribe prominent critics as fundamentally religious. He makes a strong case that modern unbelief has contained more faith than imagined in our standard histories.

The story unfolds in mostly Protestant terrain. Like many secularization narratives, it begins with the Reformation, but with a twist. Erdozain's Luther was both a prophet of burning conscience and the architect of a new hard orthodoxy. He attacked the papacy in the name of Christian conscience, only to turn to the coercive power of church and state to discipline those who deviated from his understanding of the Gospel: “His mature theology eroded the rights of conscience to the point of annihilation” (31). Faith trumped love; assent to orthodox propositions became the measure of true religion. Thus Luther put in motion a conflict between conscience and creed that has been waged ever since.

Radical Protestants play a vital role in The Soul of Doubt. Erdozain finds a sixteenth-century “Enlightenment” among spiritual rationalists like Sebastian Franck, who appealed to the spirit within, and Sebastian Castellio, whose disgust at Calvin's burning of the anti-Trinitarian Servetus led him to link theological rigor (“the folly of Alpha males” [59]) to abusive power, reduce religion to the love of God and neighbor, and call for a reasoned skepticism in the face of disputed doctrines. One of the book's most striking chapters depicts Spinoza as a heretical Christian. This idea is not wholly new, but Erdozain goes beyond Spinoza's well-known connections to the Collegiants and Quakers to place him within a broader group of English and Dutch radicals who opposed high Calvinism with theologies of divine immanence. Another canonical figure who gets a make-over is Voltaire. Erdozain maintains that the reputed archenemy of supernatural religion was in fact a “profoundly Christian thinker” who coupled his assault on superstition and dogma with a “hushed appreciation” of Jesus Christ, the man of peace, and a search for a theology of grace and redemption (8, 121, 163). This angle on Voltaire is not without precedent, either, but Erdozain develops it cogently with rich textual evidence.

Erdozain takes his story up to the second half of the nineteenth century. A chapter on Victorian controversies over Darwinism argues that the alleged warfare of science and religion had less to do with conflicting truth claims than with an assault on clerical control pursued by activists like John Tyndall and T. H. Huxley and echoed in other cultural registers by the likes of Francis Newman and George Eliot. “Calvin played a larger part in the Victorian crisis of faith than Darwin,” Erdozain puckishly asserts (175). The book ends with Ludwig Feuerbach and Karl Marx, who both appear as deeply Christian atheists. In unmasking God as a human projection cloaking egoism, Feuerbach sought “a pure and honest atheism that would restore the Christian virtue of love” (240) even as he laid the foundation for Marx's prophetic critique of religion as ideology sanctioning oppression. Boiled down, the charge of all the unbelievers we encounter here was that of hypocrisy. Christianity failed to live up to its own ideal of universal charity—Christendom was not Christian enough.

There is not a dull moment in this book, thanks in part to its marvelous prose, whose fervor often matches that of its subjects. The argument that the fiercest attacks on religion were fired by “the hottest piety” (84) has been made in various contexts before, but Erdozain connects the dots across nearly four centuries, piling up a huge mound of evidence to show just how thick were the lines of continuity among the critics. The greatest feat of The Soul of Doubt is to have identified and explicated a broad and persistent tradition of religiously based anti-orthodoxy that has often eluded students of secularization and historians working on more narrowly defined epochs.

Erdozain makes a key contribution to our understanding of secularization, but the wider import of the book is obscured by an apparent decision to bypass most of the recent work on the topic. Erdozain frames his story as largely complementary to Charles Taylor's A Secular Age (Belknap Press of Harvard University Press, 2007)—an “inner history of alienation” counterpoised to “an outer history of cooling conditions” (5)—but taken alone, his account veers toward a monocausal explanation. We briefly encounter Jonathan Israel in connection with Spinoza, but his bigger argument about the Radical Enlightenment is left aside, as is Brad Gregory's The Unintended Reformation (Belknap Press of Harvard University Press, 2012) and the newer scholarship on the Religious Enlightenment (by J. G. A. Pocock, David Sorkin, Jonathan Sheehan, and others), much of which addresses theological critiques of orthodoxy. Histories of atheism (e.g., by Alan Charles Kors, Michael Buckley, and Gavin Hyman), which stress the dependence of modern unbelief on philosophical and theological polemics, also go largely unnoticed, and David Hempton, who posits multiple causes of “evangelical disenchantment” in his Evangelical Disenchantment (Yale University Press, 2008), appears only in the endnotes. It would not have detracted from Erdozain's argument had he located his thesis in relation to this larger constellation of work, though at points its implied claims to originality might have been scaled back a bit. Nonetheless, The Soul of Doubt is unlikely to stay out of the scholarly fray for long. Owing to its quality and provocative thesis, it is bound to be dragged into debates about secularization for years to come.