Jewish existentialism is a category of work by Jewish authors dealing with existentialist themes and concepts (e.g. debate about the existence of God and the meaning of human existence), and intended to answer theological questions that are important in Judaism. The existential angst of Job is an example from the Hebrew Bible of the existentialist theme. Theodicy and post-Holocaust theology make up a large part of 20th century Jewish existentialism.
Examples of Jewish thinkers and philosophers whose works include existentialist themes are Martin Buber, Joseph B. Soloveitchik, Lev Shestov, Franz Rosenzweig, Hans Jonas, Emmanuel Levinas, Hannah Arendt, Rabbi Abraham Joshua Heschel, and Emil Fackenheim.
Jewish existentialism finds its roots in both the traditional philosophical school of existentialism and the peculiarities of Jewish theology, Biblical commentary, and European Jewish culture. Existentialism as a philosophical system grew as a result of the works of such non-Jewish thinkers as Søren Kierkegaard, Friedrich Nietzsche, Albert Camus, and Martin Heidegger.
The Books of Ecclesiastes and Job, found in the Hebrew Bible and often cited as examples wisdom literature in the Hebrew Biblical tradition, both include existentialist themes. The Book of Job tells the story of Job, who is beset by both God and satan by many hardships intended to test his faith. He ultimately keeps his faith and receives redemption and rewards from God. The Book of Job includes many discussions between Job and his friends, as well as between Job and God concerning the nature, origin, and purpose of evil and suffering in the world. The Book of Ecclesiastes is broader in scope and includes many meditations on the meaning of life and God’s purpose for human beings on Earth. Passages in Ecclesiastes describe human existence in such terms as “all is futile”  and “futile and pursuit of wind.” Much Biblical scholarship and Talmud exegesis has been devoted to exploring the apparent contradiction between the affirmation of an all-powerful God’s existence and the futility, meaningless, and/or difficulty of human life. Judaism’s treatment of theodicy makes heavy use of the Books of Job and Ecclesiastes.
Some of the trends in the modern philosophy of existentialism come from concepts important to early rabbinic and pre-rabbinic Judaism. William Barret’s Irrational Man, which traces the history of existentialist thought in the Western world, explains how the competing worldviews of Greco-Roman culture and Hebrew/Jewish culture have helped shape modern existentialism. Barrett says that the Hebraic concept of the “man of faith” is one “who is passionately committed to his own being.” The Hebrew “man of faith,” Barrett says, trusts in a God who can only know through “experience” and not “reasoning.” Juxtaposed with the believing Hebrew is the skeptical Greek “man of reason” who seeks to attain God through “rational abstraction.” The Greek invention of logic and the tradition of rational philosophical inquiry contributed to Existentialism. The Greeks invented philosophy as an academic discipline and as a way to approach the problems of existence, eventually resulting in the philosophical works of Nietzsche, Heidegger, Sartre, Kierkegaard and other existentialists. Hebraic thought trends had much more of an influence on the important concepts of existentialism. Much of modern existentialism may be seen as more Jewish than Greek.
Several core concepts found in the ancient Hebrew tradition that are often cited as the most important concepts explored by existentialism, for example, the “uneasiness” “deep within Biblical man,” also his “sinfulness” and “feebleness and finiteness.” While “the whole impulse of philosophy for Plato arises from an ardent search for escape from the evils of the world and the curse of time,” Biblical Judaism recognizes the impossibility of trying to transcend the world entirely via intellectualism, lofty thoughts, and ideals. As the late Jewish existentialist Rabbi Joseph B. Soloveitchik (b. 1903-1993) articulates for a popular audience of secular Jews, “the idea of holiness according to halakhic [Jewish law] world view does not signify a transcendent realm completely separate and removed from reality…of the supreme good…the halakhic conception of holiness…[is] the holiness of the concrete.” In the words of Barret, “right conduct is the ultimate concern of the Hebrew,” and indeed for the observant Jew, according to R. Soloveitchik. Therefore, the Jewish tradition is differentiated from the Greek system of thought, which emphasizes correct knowledge, thinking, and consciousness as the passports to transcendence of the physical world. Some traditions of ancient Gnosticism, like the neo-Platonist desert cults, also subscribed to an idea similar to the Platonist ideal of “true knowledge of the Good” being a gateway to transcending one’s ordinary, physical existence.