Abraham Joshua Heschel wrote extensively on Jewish existentialist themes. Among his many works on Jewish theology are the books The Sabbath (1951) and Who is Man? (1965). The best-selling The Sabbath explores the concept of the Jewish Sabbath (Shabbat) and its significance as a period of heightened connection between God and his creation of Man. Heschel’s The Sabbath is also well known for the concept of the Shabbat as a “cathedral in time” (rather than in space, as cathedrals are in the Christian tradition). For Heschel, “The Sabbath arrives in the world… [and] eternity utters a day.”
In Who is Man? Heschel explicates his thesis that Man is a being whose ultimate purpose and task in life is to wonder about existence, to ponder and pine for his Creator. In his words, “Man is a being in search of significant being, of ultimate meaning of existence.” In Who Is Man?, Heschel also constructs a famous dichotomy between “biblical man” and “ontological man.” Heschel’s concept of the “ontological man” is an explicit response to Heidegger’s ideas about Dasein, which for Heschel is a human who merely exists passively, rather than lives actively as human in the world. A further difference between "biblical" and "ontological" man is that "ontological" man is stuck on basic questions of ontology (the study of the nature of being and existence) and only “seeks to relate the human being to transcendence called being” whereas the "biblical man" “realizing that human being is more than being…seeks to relate man to a divine living, to a transcendence called the living God.” Heschel critiques Heidegger’s stance toward seeking an understanding of Being as the ultimate reality without reaching out to a higher power while at the same time living actively in the real world (as “biblical man” does), saying,
“…simply to ‘surrender to being,’ as Heidegger calls upon us to do, he would…reduce his living to being. To be is both passive and intransitive. In living, man relates himself actively to the world…The decisive form of human being is human living…to bring into being, to come into meaning. We transcend being by bringing into being---thoughts, things, offspring, deeds.” ( Who is Man?, 94-95)
Heschel’s work deals with man’s relation to God and man’s ability to make meaning in his own life through the sanctification of certain traditions, ideas, and time periods. Heschel’s books (especially Who Is Man?) are primarily concerned with the existential question of the purpose and meaning of human life, which is one of the foundational questions of theology concerning the relationship between human beings and God.
Heschel is also reacting to Nietzsche’s secular existentialism in Who Is Man? In reaction to Nietzsche’s assertion that man must make meaning for himself by his “will to power” in an indifferent universe, Heschel cites human being’s obsession with finding meaning outside of themselves as evidence of the existence of a higher being. He says, “To be overtaken with awe of God is not to entertain a feeling but to share in a spirit that permeates all being.” For Heschel, man’s proclivity to be in awe of God is an important part of the make-up of all humans. He can be said to be an "experiential Jew" concerned with the interior experience of God as the primary mode of popular religious experience. Rabbi Soloveitchik would call Heschel an “homo religiosus.” Heschel is also reacting to Kierkegaard and Nietzsche’s secular existentialism in Who Is Man? Heschel can be said to be an "experiential Jew" or a “homo religious” ("religious man") “totally devoted and given over to a cosmos that is filled with divine secrets and eternal mysteries.” [
See also: Jewish existentialism