No photo of a rabbi has come to symbolise his legacy  quite as much. In 1965, Martin Luther King led a march against racial segregation in the USA that set out from Selma, Alabama; arm-in-arm with the civil rights leaders was a white-bearded figure with the aura of a sage. “I felt my legs were praying,” said Rabbi Abraham Joshua Heschel.

For Heschel, protest went hand in hand with prayer, propelled by a prophetic passion that infused his writings and speeches. He became the exemplar of tikkun olam, the call to repair the world through social action, even though he may not have popularised the Hebrew term himself. Prayer was meaningless, he once wrote, “unless it is subversive, unless it seeks to overthrow and ruin the pyramids of callousness, hatred, opportunism and falsehoods”.

Fifty years after his death, a new biography — part of the admirable Yale “Jewish Lives” series —  has been published to celebrate the life and work of a rabbi whom the former chancellor of the Jewish Theological Seminary in New York, Ismar Schorsch, described as “the most important Jewish thinker of the modern period”.

The “militant mystic”, as Time magazine dubbed him, was a born to a Chasidic family in Warsaw who named him after a revered great-great-grandfather, the Apter Rav. 

Princeton University professor Julian E. Zelizer does a fine job in evoking the different social and cultural milieux in which Heschel moved, from the often grinding poverty but religious richness of Jewish life in Warsaw to the comforts of America, where the primary threat to the Jewish community was spiritual desiccation.

Among the early influences on Heschel was the Kotzker Rebbe who, Zelizer says, “conveyed a sense of righteous indignation about the state of the world”. One of the last of his many works, written in Yiddish rather than English, was about the Kotzker.

The young Heschel proved a prodigy in his steibl schooling. But while much of Chasidic society today maintains an insular wariness of the wider world, his environment was “open to Zionism and the Jewish enlightenment” and his family were not opposed to the desire to improve his secular education that carried him first to Vilna and then toBerlin University.

In Berlin, he befriended the philosopher Martin Buber and co-edited an arts journal with the Yiddish poet AN Stencl, who later found refuge in London. But in contrast to the humanism of his university studies, German society was succumbing to a darker force. One night at the opera he saw the audience rise to their feet to greet the arrival of Hitler.

He got out of Germany only a few months before the outbreak of war, rescued by the offer of a  fellowship at the Reform Hebrew Union College in Ohio, Cincinnati. “Interdenominational fluidity was common during the interwar years,” Zelizer notes. Since the canteen food wasn’t kosher, he lived off a diet of tuna and eggs.

Institutional religion enjoyed an unprecedented boom in post-War America; the number of Conservative synagogues rose from 35 in 1945 to 800 by 1971. But Heschel, who moved to the Conservative JTS in New York, worried that the suburban Judaism of many communities was stultifying. “Our buildings are beautiful but the homes are a wilderness,” he once wrote.

In books such as The Sabbath, Man Is Not Alone and God In Search of Man, he strove to emphasise the importance of developing personal spiritual awareness. The subtitle of Zelizer’s biography, A Life of Radical Amazement, alludes to one of Heschel’s most famous phrases, highlighting his belief that a sense of wonder was integral to a religious life: a religious person should be able to feel “the hidden love and wisdom in all things”.

Zelizer traces the roots of Heschel’s activism to his frustration at what he felt was the passivity of American Jewish organisations while their fellow Jews were perishing during the Holocaust. He lost his mother and three sisters and saw himself as “a brand plucked from the fire in which my people was burned to death.” 

When he took part in a rabbis’ march in Washington in 1943 to lobby the American government to do more for their brethren abroad, it was “his first real taste of grassroots activism in the pursuit of justice,” Zelizer says. He was later to play a significant role in the Vatican’s renunciation of antisemitism in the ‘60s and in awakening American Jewry to the oppression of Jews in the Soviet Union.

In 1962, he published what Zelizer says became his “most influential and enduring work”, The Prophets. “There is immense silent agony in the world, and the task of man is to be a voice for the plundered poor, to prevent the desecration of the soul and the violation of our dream of honesty,” he wrote.

No one did more to put the prophetic tradition of justice at the heart of modern Jewish living. Acting on his words, he threw himself into the fight against racism which he condemned as “satanism”. It was not the only cause which thrust him into the national limelight. He was a vehement opponent of the Vietnam War and when he once met Henry Kissinger, then National Security Adviser, he confronted him with the question: “How could you as a good Jew prosecute a war like this?”

But he defended Israel after the Six-Day War when radicals were accusing it of being an outcrop of the imperialist West and was commissioned to write a work to explain the country’s importance to Jews, Israel: An Echo of Eternity, in which he wrote: “We are tired of apologising for our existence.”

Heschel came to fame when many religious leaders were involved with the progressive politics of the 60s. As Zelizer reminds us, society later took a turn to the right with the rise of the Moral Majority in the USA and the influence of neo-conservatism. 

But as our world is shaken by new challenges, perhaps his words will resonate once again: “There is one evil which most of us condone and are even guilty of: indifference to evil.”

Abraham Joshua Heschel — A Life of Radical Amazement - Julian E. Zelizer is published by Yale University Press, £16.99