Of the many foundational concepts that have been handed down to us from sociology, none has been more controversial than secularization. The idea – that religion would decline with the progress of modernity – can be found in various forms going back to the founders of the social scientific enterprise. Marx, Weber, Durkheim, and many others advanced some version of the notion as a core part of their theoretical projects. But the last fifty years has seen a decided rejection of secularization as a both a theory and as a description of empirical reality. Put simply, it has seemed to many social scientists that religion was not just surviving but thriving in the modern world, which undermined the entire theoretical framework.

Against this backdrop of conceptual disagreement, Beyond Doubt seeks to revisit many of the theoretical and empirical questions to show that, in fact, the core tenets of secularization theory are correct. Following Mark Chaves, Steve Bruce, Karel Dobbelaere, and others who have continued to support the theory, Kasselstrand, Zuckerman, and Cragun provide a bracing tour of the evidence for secularization in the past several decades. Religion is in decline in many parts of the world by multiple metrics, the authors persuasively show, as people no longer claim supernatural beliefs, justify their behavior by religious rationales, or belong to any particular religious institution.

The book's most significant contribution comes in its critique of the so-called supply side theory of sociologist Rodney Stark and others, a dominant interpretive approach in the last several decades. Stark and his follower have argued that that the demand for religion is constant in human societies because, as fellow sociologist Andrew Greeley once put it, “Man needs faith” (99). The claim is obviously theological rather than empirical and, as such, is seemingly unsuitable for use as a foundational social scientific proposition. But that fact has not prevented it from being widely adopted among those who study religion. Because there is a constant demand for religion, the supply side argument goes, there cannot be any real decline in the status of religion in society, only a changing shape of religion as suppliers compete in a religious marketplace and attempt to meet the constant demand. There might be a temporary dip in religious expression, as suppliers are coming up to speed or getting organized, but any decline, given the constant demand, should be regarded as illusory.

Kasselstrand et al. point out that the supply side theory, since it is really a theological idea, turns the religious state into one that is “natural, innate, and universal” (98), while construing secularity as “unnatural” and “artificial” (99). That, in turn, dismisses the hundreds of millions – the authors suggest maybe as many as a billion people – who live without religion. “Of course, it remains true that most humans are religious, and only a minority are secular,” Kasselstrand et al. point out. “But just because a minority of humans are left-handed, or have perfect-pitch, or are over six feet tall, or are monolingual, or are illiterate, or are homosexual, or are vegetarian, or are colorblind, or have 20/20 vision, or are secular, does not make any of these traits, characteristics, or orientations unnatural, artificial, or manifestly untenable” (105). The point is that, regardless of the current minority status of those who live without supernatural consolation, there is a clear vector of development that has emerged: a trend toward the differentiation of religion from other aspects of society and the resulting privatization or outright rejection of supernatural claims as modernization progresses.

The authors rightly acknowledge the traditional limitations of social science data, which might bother some more qualitatively minded readers. There is in that data a well-known Protestant/Christian bias in the sorts of questions asked and, in some cases, in the interpretation of those questions. Asking how many times a day a person prays, for example, and then judging their level of religiosity based on their answer, only makes sense if you first control for the relative importance of prayer in a particular tradition. A Christian who prays three times a day would be considered quite religious. A Muslim who prays three times a day, rather than the five times mandated in the Qur'an, would be considered lax.

Given the bias in that data, there were times in reading the book when it felt like the whole secularization debate was an argument among Christians and post-Christians. The authors spend time on Korea as a case study, for example. But while mentioning the decline of Buddhist belief and affiliation in passing – one of the few references to Buddhism that I found in the book – the case study is mostly concerned to explain the recent rise in Christian affiliation in the country before an abrupt decline in the last several years.

And yet in spite of these limitations, this is a compelling book and a shot across the bow to the many who still seek to deny the fundamental reality of secularization. As the authors conclude, “The evidence for secularization is clear. Secularization is happening. Secularization is real. It's beyond doubt.”