“I and Thou,” a short treatise by the Jewish theologian Martin Buber, was published in German in 1923; by the time it appeared in English, fourteen years later, the translator could already call it “one of the epoch-making books of our generation.” When Buber died, in 1965, his Times obituary focussed mainly on this one book, crediting it with making Buber “a pioneer bridge builder between Judaism and Christianity.” Buber’s philosophy of dialogue had been enthusiastically embraced by such Protestant thinkers as Reinhold Niebuhr and Paul Tillich. Even today, “I and Thou” remains a staple of religion courses and bookstore spirituality sections, and inspirational quotes from it—“An animal’s eyes have the power to speak a great language”—circulate endlessly on social media.

Yet “I and Thou,” which uses a generalized, ecumenical vocabulary, has never enjoyed the same stature among Jewish readers as it has with the world at large. After Buber’s death, the novelist Chaim Potok observed, “It was a source of considerable anguish and frustration to Martin Buber that he was more appreciated by Christians than by Jews.” This was a painful irony, since few people in the twentieth century had thought more passionately about Judaism and Jewishness. Buber had written dozens of books about Jewish history, theology, mysticism, and scripture. He was an early adherent of Zionism, worked on translating the Hebrew Bible into German, and popularized Hasidic folklore; during the Nazi period, he ran a Jewish adult-education program in Germany, to sustain the morale of his persecuted people. To Jewish historians, this is the Buber who matters: the writer and teacher whose career spanned the most important events of Jewish modernity, including the end of German-Jewish civilization and the creation of the State of Israel, where he spent the final decades of his life.

“Buber was a contested figure,” Paul Mendes-Flohr writes in his new biography, “Martin Buber: A Life of Faith and Dissent” (Yale). “He evoked passionate, often conflicting opinions about his person and thought.” There were always readers who distrusted Buber’s thinking about Judaism, which was defiantly innovative and anti-traditional. Some people questioned whether he really was a major thinker or just a charismatic impresario of ideas. In the nineteen-twenties, when Judah Magnes, the chancellor of the newly founded Hebrew University, in Jerusalem, tried to hire Buber as a professor, the faculty repeatedly refused to accept him, considering him not quite a true scholar. Only in 1938, as Buber tried to leave Nazi Germany, was a chair found for him—not in religion or philosophy but in the sociology department. The snub was hard for him to bear, and he accepted the appointment only after much internal struggle.

Reading Buber, it’s not hard to understand why he might inspire suspicion. His prose, shaped by the literary tastes of the early twentieth century, tends to be high flown rather than precise. His book “Daniel” (1913) is written in a rhapsodic style that owes something to Nietzsche’s “Thus Spake Zarathustra” and something to Symbolist poetry: “Because we cannot circle above all existence—sleepless, unbroken, boundless, glowing—we content ourselves with being submerged and awakening.” Even some of his admirers admitted that they couldn’t always be sure of what he was trying to say. (“I have read it to the end and—understood nothing,” Magnes wrote after reading “Daniel.”) The American translator of “I and Thou,” Walter Kaufmann, acknowledged that Buber “tends to blur all contours in the twilight of suggestive but extremely unclear language. Most of Buber’s German readers would be quite incapable of saying what any number of passages probably mean.”

Such haziness was inevitable, because the questions Buber was trying to answer were the most ineffable ones of human life: What is the meaning of our existence? How can we achieve the feeling of wholeness that we so painfully lack? Above all, Buber asked, how do we find our way to God, now that religious belief has become so challenging for modern, educated people? Anyone who believed it was possible to give crystal-clear answers to such questions would have to be a messiah or a charlatan, and Buber was neither.

At the heart of Buber’s theology was his theory of dialogue—the idea that what matters is not understanding God in abstract, intellectual terms but, rather, entering into a relationship with him. Such a relationship, he believed, is possible only when we establish genuine relationships with one another. “Whoever goes forth to his You with his whole being and carries to it all the being of the world, finds him whom one cannot seek,” he wrote. In daily life, we usually fail to live up to this ideal. We tend to treat the people and the world around us as things to be used for our benefit. Without this mind-set, which Buber called “I-It,” there would be no science, economics, or politics. But, the more we engage in such thinking, the farther we drift from “I-You,” his term for addressing other people directly as partners in dialogue and relationship. Only when we say “You” to the world do we perceive its miraculous strangeness and, at the same time, its potential for intimacy. Indeed, it’s not only human beings who deserve to be called “You.” As Buber wrote, even a cat or a piece of mica can summon up in us the feeling of a genuine encounter with another: “When something does emerge from among things, something living, and becomes a being for me . . . it is for me nothing but You!”

This way of thinking about God and faith may seem to be remote from Judaism as most Jews traditionally understood it. But, in a sense, Buber’s rejection of Jewish orthodoxy made him a good representative of his generation of German-speaking Jews, many of whom turned decisively away from Jewish practice. Buber was born in 1878 in Vienna. The course of his life was changed when he was three years old, when his mother ran away with a Russian officer, leaving without saying goodbye to her son. Mendes-Flohr emphasizes that this early loss left Buber with a lifelong feeling of abandonment, which in turn fed and shaped his religious longings. The God he describes in his work is neither a stern lawgiver nor a merciful redeemer but a close presence to whom we can always turn for intimacy. “That you need God more than anything, you know at all times in your heart,” he wrote. “But don’t you know also that God needs you—in the fullness of his eternity, you?”

His mother’s abandonment determined Buber’s fate in a more concrete way, too: he was sent to live with his paternal grandparents, in the city then known as Lemberg (today’s Lviv), the capital of the province of Galicia, in the Austro-Hungarian Empire, and they raised him until he was an adolescent. His grandfather Salomon Buber was a wealthy philanthropist and a Jewish scholar of renown; Martin grew up in an observant household and was educated in Hebrew and in Yiddish, as well as in German. But, when he was fourteen, he moved in with his father, who had remarried and moved to Lemberg. This household was more secular and assimilated, and Buber stopped observing most Jewish customs. By sixteen, he was ripe for rebellion: he recalled feeling “a raging hatred of the entire nauseating atmosphere in which I lived, a wrathful aversion against the official morality, the official education.”