DIETRICH BONHOEFFER, born a century ago today, is one of the best-known Christians of the 20th century. He is one of ten martyrs commemorated above the door of Westminster Abbey, his family home is now a museum and several tourist companies offer “pilgrimages” to sites with which he was associated. He may be the closest thing modern Protestants have to a saint.

If Bonhoeffer were merely the object of a “cult” or model for piety, marking the centenary of his birth could be kept behind church doors. But Bonhoeffer’s legacy raises unsettling questions about the relationship between faith and politics. Bonhoeffer’s Christian convictions leach into political action, dissolving easy distinctions between the two.

In 2002, in a speech thanking the Bundestag for German support in the War on Terror, President George W. Bush held Bonhoeffer up as a model of fighting tyranny. Paul Hill, an American executed in 2003 for the murder of an abortion doctor and his guard, disturbingly compared Bonhoeffer’s opposition to Nazism with his own actions. Such appropriations are not new: since his death, Bonhoeffer’s reputation has been conscripted by all kinds of political causes.

Religious leaders in East Germany used his story to prove the Church’s anti-Fascist credentials. Opponents and supporters of wars in Vietnam, the Falklands and Iraq used him to shore up their case. And in the country which executed him for treason, cities have vied to honour him by naming streets and schools after him.

This ought to astonish us much more than it does, for the values that shaped him were not those of modern liberal democracy, but the cultured elitist values of a Germany now long forgotten and the thoughtful, deeply held religious convictions to which secularisation makes most Europeans indifferent or suspicious.

When Dietrich Bonhoeffer was born on February 4, 1906, in Breslau (now Wroclaw in Poland) he entered a Germany in which Bismarck’s landowning class still had politics in its grip. Beneath the surface seethed the pressures of rapid industrialisation and population growth. Yet even when Berlin collapsed into chaos at the end of the First World War this did not impinge greatly on Bonhoeffer’s privileged family. He grew up in the educated milieu of Berlin University where his father held, from 1912, the respected chair of psychiatry.

Against the instincts of his empirically inclined family, Dietrich studied theology, gained his doctorate and had qualified as a university lecturer by the time he was 24. After a “gap year” in New York he was ordained and settled to a career teaching theology.

When Hitler became Chancellor in 1933, Bonhoeffer knew immediately that this was a crisis for Germany, for Europe and for himself. He feared that the popularity of the Nazis threatened decency, law and order.

In the beginning Bonhoeffer put his faith in the Church as a vehicle for opposition. Though junior, he helped to mobilise opposition to state interference in church life. He attempted — without success — to persuade the nascent World Council of Churches to reject the Nazi state church as heretics, and began to feel frustration with his Church’s reluctance to stand up for the Jews.

In 1935 he was asked to head a seminary training pastors for the illegal Confessing Church, established in opposition to the pro-Nazi state church. He thrived in these unusual circumstances, writing two of the books that have made his name. Life Together (1938; English 1954) reflects on the life of his seminary, and The Cost of Discipleship (1937; English 1959) argues that “cheap grace is the mortal enemy of our Church. Our struggle today is for costly grace.”

In 1936 the Gestapo began to move against him. It withdrew his teaching licence, forbade him from speaking in public or publishing, banned him from visiting Berlin and closed his seminary. In 1939 Bonhoeffer accepted an invitation to lecture in the US but realised he could not live out the war in America. He returned to Germany on the eve of war and agreed, on the urging of his brother-in-law — the senior legal officer in German military intelligence — to join the anti-Nazi conspiracy.

In the early years of the war Bonhoeffer worked as a double agent. Ostensibly working for military intelligence, he used his church contacts to sound out responses to a possible coup. His texts, published posthumously as Ethics (1949; English 1955), debate with breathtaking originality how Christians might conform themselves to Jesus Christ in a postwar, non-religious age.

A message conveyed by Bonhoeffer through Bishop George Bell to the British Foreign Secretary was dismissed by Anthony Eden in a marginal note: “I see no reason whatsoever to encourage this pestilent priest.” In 1943 Bonhoeffer was arrested on minor charges and imprisoned until evidence linked him with the attempt on Hitler’s life in July 1944. He was moved several times, summarily tried, and hanged with his co-conspirators, General Hans Oster and Admiral Wilhelm Canaris, at Flossenbürg concentration camp on April 9, 1945.

Little known during his life, Bonhoeffer’s posthumous publication won huge recognition, placing him in the first rank of 20th-century theologians.

His decision to join the resistance was not easy. He had been a pacifist since forming a close friendship with a Frenchman in America who, like him, had lost a brother in the First World War. Bonhoeffer had written a book on the Fall; he understood that killing Hitler was murder and that murder was a sin. But he believed that inaction was also sinful. He never renounced pacifism and refused to justify his actions by arguing that the death of one individual might save the lives of many.

Bonhoeffer’s class gave him a sense of duty, and the urbane and humane culture of his family home gave him ground beneath his feet. In this he was no different from the men with whom he was tried, and who died for the same crime without religious convictions.

So what was the difference? What is the role for Christian convictions in politics? In a poem, Christians and Pagans, smuggled by Bonhoeffer from prison, he reaches towards an answer. In need, all people seek mercy and bread, whether Christian or not. And God hangs on the Cross for everyone, irrespective of their belief. The difference? “ . . . tormented by sin, weakness and death, Christians stand by God in his agony.”

Stephen Plant teaches theology in Cambridge and is the author of Bonhoeffer (Continuum, 2005).