If there was one thing the sages of the 19th century agreed on, it was that the 20th century would not be an age of religion. The most enlightened and the most reactionary minds of fin-de-siècle Europe saw no place for traditional Christianity in a culture battered and exhilarated by the revelations of Darwin and the provocations of Nietzsche. To noble spirits like George Eliot and Thomas Hardy, the death of belief was a cause for mourning, and the spur to a new, humanistic morality. Others, less wise or less scrupulous, thought the God-shaped hole could be filled by Marxist revolution, or the idols of nation and race.

But few advanced thinkers, on the eve of 1914, would have predicted that, at the dawn of the 21st century, traditional religion would turn out to be more durable, more popular, and more honorable than any of its would-be supplanters. How this came to pass is the story Michael Burleigh tells in “Sacred Causes” (HarperCollins, 576 pages, $27.95), his wide-ranging and brashly opinionated new study. Mr. Burleigh’s previous book, “Earthly Powers” (2006), dealt with the rise of secular utopian ideologies in the 19th century. Now, in a series of linked case studies — from Leninist Russia to Northern Ireland during the Troubles — Mr. Burleigh examines how those ideologies tried, and failed, to usurp the authority of religion during Europe’s catastrophic century.

The challenge to religion, in Mr. Burleigh’s telling, was threefold. First, starting in 1917 with the Bolshevik Revolution, was communism, whose materialist view of history could find no way of understanding religion except as an “opiate of the people.” Like the Jacobins who were their direct inspiration, the Bolsheviks felt the need to stage their repudiation of religion in unmistakably public ways. The tombs of saints were opened to show the peasants that holy bodies were not, in fact, incorruptible; Leningrad’s Kazan Cathedral was turned into the Museum of the History of Religion and Atheism. Hand in hand with these symbolic humiliations went the brutalities the Bolsheviks practiced in every sphere: Thousands of priests were kidnapped, imprisoned, or summarily shot.

All this, however, was only preparatory destruction, the prelude to what the communists hoped would be the creation of a new political religion. Mr. Burleigh reminds us that the personality cults around Lenin and Stalin were explicitly blasphemous, with the dictators elevated into all-seeing, all-providing gods. Here, of course, communism overlapped with Nazism, its ostensible opposite. Hitler, too, was cast as a replacement for God, sometimes explicitly, as in a 1937 painting of a haranguing Führer titled “In the Beginning Was the Word.”

In Nazi Germany, however, and still more in fascist Italy, the dictators did not seek to annihilate religion. Instead, they wanted to bully it into submission and steal its legitimacy. Ironically, this meant that Lutheran and Catholic clergy had more painful choices to make than their Orthodox equivalents, who were simply condemned to destruction. Mr. Burleigh, best known as a historian of Nazism, devotes his most substantial chapters to the tortuous dance of compromise and resistance followed by the German and Italian churches.

He is especially concerned to vindicate the reputation of Pope Pius XII, and disputes the charge, made by John Cornwell that he was “Hitler’s Pope.” On the contrary, Mr. Burleigh shows, neither the Nazis nor anyone else in the 1940s doubted that the Vatican was opposed to Hitler. Many bishops, including the future Pope John XXIII, and lower clergy acted heroically and effectively to rescue Jews from the Holocaust — though their first concern tended to be for Jewish converts to Catholicism. If Pius never committed the whole weight of the Papacy to the fight against the Nazis, Mr. Burleigh argues, it is because he thought such a condemnation would endanger Catholics in Nazi-occupied lands, and destroy the Vatican’s last shreds of influence over Germany’s actions.

Mr. Burleigh is sympathetic to the Vatican’s difficulties in “balancing its diplomatic cum spiritual objectives with the role of moral prophecy.” His Vatican-centered approach, however, does not prevent him from pointing out the important role played by Catholic clergy in fascist or quasi-fascist regimes in Vichy France, Spain, and Croatia. In Slovakia, indeed, the dictator of the Nazi puppet state was a priest, Jozef Tiso. The pains it clearly costs Mr. Burleigh to record such examples of collaboration are balanced by the pride he takes in the role of the Church during the Cold War, when the heroism of Pope John Paul II helped to overthrow the Soviet empire.

Mr. Burleigh’s chapters dealing with these appallingly difficult moral questions are well-researched and earnest. It is odd, then, to turn to the latter part of the book, when his focus turns to Western Europe after the wars, and to the third major challenge to religion: secularism. When Mr. Burleigh deals with this less fraught and more contemporary issue, the scholar gives way to the slashing pundit. He clearly enjoys lashing out at plump targets like New Age gurus, campus leftists, politically correct politicians, and trash pop culture. Yet he never grapples with the question of why Western Europe has become so tepid in its Christian faith, or fairly weighs the spiritual gains of the post-1960s West against its losses. The advent of feminism and gay rights, and the decline of racism, are surely moral victories that the contemporary West can count to its credit.

Instead, Mr. Burleigh, a professor himself, devotes an understandable but disproportionate amount of energy to attacking the feckless professoriate, who starting in the 1960s “inhabited a purely transtemporal space where the quest for vicarious rejuvenation often meant remaining juvenile into one’s retirement, sometimes manifested through vampiric interest in female students.” This kind of amusing but unserious attack proliferates toward the end of “Sacred Causes,” as in the chapter on the Irish Troubles, which includes a virtual standup comedian’s rant about the delinquencies of the Irish. (Irish builders, Mr. Burleigh writes, are “bodgers and shysters,” summoning up a vision of the author in his half-remodeled kitchen, cursing the contractor.)

That entire chapter feels extraneous to the book, a merely competent rehash of well-known recent events, as does the concluding chapter on Islamist terrorism after September 11, 2001. Altogether, “Sacred Causes” would have benefited from more disciplined writing and a more focused argument. It is a tribute to Mr. Burleigh’s intellectual passion that, even without them, he manages to challenge and enlighten.

akirsch@nysun.com