It is not possible, he believes, to build a sustainable social order around a collection of desiring subjects—yet the strength of the ideologies of the 1960s is too great to permit an alternative to emerge. For Europe, beset by a globalization that threatens its coherence and independence from the world’s superpowers, the only answer is to return to its roots. For Mr. Roy, those roots are Europe’s Christian heritage and the tradition of pre-1960s liberalism grounded in the enlightenment and classical ideas of civic and republican virtue.

American Christians are an individualistic lot. When they ask, “Is Europe Christian?”—the title of Olivier Roy’s book, translated from the French by Cynthia Schoch—they tend to assume the way to answer it is to determine how many Europeans believe in the Christian message. The percentage of Christians in Europe will tell you how Christian Europe is.

That is not Mr. Roy’s approach. Christianity in Europe has long been a public, political and legal force. Europe has a long history of state churches. In many European countries, leading political parties were formed with clerical support and have, or used to have, an explicitly Christian view of politics. While Italy’s Christian Democracy party collapsed after the Cold War, in Germany the Christian Democrats, in alliance with the Bavarian Christian Social Union, dominate the ruling coalition.

So Mr. Roy, a French political scientist now teaching in Italy and one of the most accomplished students of religion and politics in our time, is not primarily focused on how many Europeans are Christian believers; he is asking whether Christian institutions, values and ideas remain central to European politics, culture and law. His answer is that Europe is rapidly becoming less Christian but that the problem is not, as many think, the rise of Islam. Both faiths, Mr. Roy believes, are fighting a seemingly irresistible trend toward the marginalization of religious concerns in a pleasure-seeking secular culture.

Europe’s secularizing trends are old. After the Peace of Westphalia in 1648, the continent was permanently divided into Protestant and Catholic sections. This enshrined a pragmatic conception of the political order rooted in power politics, replacing the older idea of Europe as “Christendom,” united around a common faith. The Enlightenment and the rise of liberal ideas further weakened the hold of Christianity on European institutions and culture. The 19th-century rise of anticlerical and, in some cases, anti-Christian political parties put Christian churches even more on the defensive.

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