In the last decades of his life, he was deeply involved in various public issues which gave him a high profile and evoked great admiration, especially among the younger generation. Active in interfaith understanding, he visited Rome during the Second Vatican Council for meetings with Cardinal Agostino Bea, in connection with the formulation of the document of Catholic relations with the Jews. His social-ethical teachings found practical expression in his involvement with the civil rights movement. He marched with Martin Luther King, Jr., and participated in protest marches and demonstrations to secure equal rights for U.S. blacks and to end the U.S. military intervention in Vietnam.
In October 1938 he was expelled from Germany together with all other Polish Jews who did not hold German citizenship. After teaching for a time in Warsaw, he left Poland on the eve of the war and, after a few months in England, reached the United States, in 1940, to join the faculty of Hebrew Union College in Cincinnati. He remained there for five years and then moved to New York, where, until his death, he was professor of ethics and mysticism at Jewish Theological Seminary.
His books cover many aspects of Jewish thought. He sought to create a synthesis welding traditional piety and learning with the rational thought and scholarship of western Jewry. He believed that the essence of Judaism lies in the collective memory of the Jewish people, with the Bible as the source of certainty. This relates not only to man’s search for God but also to God’s search for man, as reflected in his book titles "Man's Quest for God" and "God in Search of Man".
He was deeply influenced by western thought, especially Neo-Kantianism, but insisted that Jewish thought cannot be simply fitted into philosophical categories and that God is more than a postulate of reason. Judaism, he believed, teaches a living relationship between man and God and true Jewish observance is expressed when God’s concern for His creatures evokes, in man, a response of love and devotion. The experience of wonder is crucial to true religious expression, leading to an awareness of the grandeur of God.
Heschel stressed the holiness of time in Jewish thought and practice. Judaism, he noted, has not created holy places, and unlike modern technological civilizations does not emphasize space, but hallows times, notably the Sabbath and festivals.
His writings are aphoristic, poetic, and sensitive. His theological works include "Man Is Not Alone" and "Who Is Man?" (a defense of human dignity). "The Prophets" (based on his doctoral thesis) seeks to analyze the prophetic consciousness. "The Sabbath" is a paean to the Sabbath day, while "The Earth Is the Lord’s" is a poignant evocation of the lost world of east European Jewry. The broadness of his interests is illustrated by his two books on great Jewish thinkers: the rationalist Maimonidesand his last work on the Hasidic teacher, Menahem Mendel of Kotsk.