The principal thesis of this book is that the contemporary secular state suffers from a legitimacy gap. When political power ceased to be nested within religion and began to claim independent authority, it needed a source of legitimacy to replace religion. During the modern era, that alternative source has become the democratic ideas of the people's will and individual rights. Depaigne associates that shift in forms of legitimacy with Max Weber's thinking on the shift from traditional to legal-rational authority. He also views it as a shift from ‘substantive’ to ‘procedural’ forms of legitimacy. Whereas religions provided societies with substantive values through, for example, divine or natural law, democracy and human rights provide no more than procedures through which the people's good, understood as the people's will, could be expressed. Procedural institutions are therefore ‘empty vessels’. They are also universalist vessels, devoid of the particularism that segments humanity into different communities.

Where, then, is the legitimacy gap? Depaigne presents it primarily as a missing foundation: if secular authority is grounded in law, as Weber suggested, that law, including constitutional law, must depend upon something external to itself for its legitimacy. That foundation, we might suppose, is provided by the will of the people; but a people can be a people and can express its will only after it has been constituted politically as a people. Thus a legitimacy gap remains at the secular state's foundation. Depaigne interprets social contract theories, particularly those of Hobbes and Rousseau, as efforts to fill the gap; but those also prove inadequate. Without the particularities provided by religion and culture, how can we discern the particular people whose members have notionally contracted to be a people? Depaigne also reckons that, without the ties and identities provided by culture and religion, universalist procedural institutions will be too insubstantial, too ‘thin’, to command the allegiance that a state requires.

We might infer from all this that Depaigne wants to throw secularisation into reverse. He does not; but he rejects the simple idea that secularisation consists in a state's separating itself comprehensively from religion or from culture in general. Secular states still need the support of religion and culture and cannot escape involvement with them. Depaigne devotes the second part of his book to elaborating three constitutional models of secularisation: a state that strives for separation from religion; a state that manages a plurality of religions by remaining neutral among them; and a state that provides asymmetrically for religions by sustaining an established religion but also allowing the practice of other faiths. He exemplifies these models in chapters on, respectively, France, India and Malaysia. These concise but informative chapters illustrate how secularisation has had to adjust to the particular histories and circumstances of different societies; they also indicate how these soi-disant secular states have remained entangled with religions and have sometimes engineered religious reform.

Depaigne's book is based on extensive and wide-ranging scholarship and is ambitious and adventurous in scope. It is also replete with bold theses and grand generalisations. Almost inevitably, that makes it controversial. There is less to quarrel with in the book's second part, since the author's different models of secularisation are largely empirical in inspiration and are shored up by evidence. That part also differs in focus from the first: religion figures less as a possible source of legitimacy than as an object of more or less legitimate treatment by the secular state.

It is the book's first part, alleging a ‘legitimacy gap’, that invites a more sceptical response. If secular states suffer from that gap, why are they not crumbling around us? Depaigne's unsatisfactory answer is that religion and culture have moved in to fill the gap. Why ‘unsatisfactory’? Because Depaigne offers a sociological solution to what he presents as a logical and normative problem; because he supports his essentially empirical answer with little evidence; because he seems to take for granted that religion (any religion?) can and will legitimate political power; because he moves easily between the religious and the cultural, as though their difference were of little consequence; and because his appeals to sprawling and indiscriminate notions of ‘religion’ and ‘culture’ really tell us very little. ‘Secularism’, he advises us, ‘should be about using available religious and cultural references in a given community as a resource to provide content and legitimacy to its political institutions’ (p 191); that sounds uncomfortably close to the advice offered by Machiavelli in his Discourses. Readers of this book should expect to be provoked as well as stimulated.