Emphasis placed on human free will and what is true to an individual.
Reprinted from The Jewish Religion: A Companion, published by Oxford University Press.
Existentialism is the intensely personal philosophy in which the individual responds not to a philosophical system, which he surveys from the outside, but to what is true for him. In religious existentialism, the believer adopts monotheism not because his reason has demonstrated that there is a God but, as Kierkegaard puts it, by a "leap of faith."
Heschel's suggestion that Judaism substitutes a "leap of action" for the leap of faith sounds good but is unhelpful from the philosophical point of view in that it ignores the question of belief upon which, presumably, the action is founded. The two best-known Jewish religious existentialists are Martin Buber and Franz Rosenzweig. These two have been followed, rather too blindly, by a number of contemporary Jewish religious thinkers whose attitude has not unfairly been dubbed "Kierkegaard with a yarmulke."
An atheistic existentialist like Sartre affirms that existence precedes essence. This means that there is no essence outside man to which he must conform. Man creates his own values, which is to say that he determines his own essence through his existence and not the other way round. The man who allows others to determine his way of life is being false to himself if he simply acquiesces in it. He must make his own individual choice if he is to be true to himself. Thus, for the atheistic existentialist there is no God who has created man and provided the pattern of human existence.
Obviously, such an attitude is totally opposed to Jewish belief. But even the religious existentialists place great emphasis on man's free choice. For them, the choice comes first. The essence or pattern provided by theistic belief is there because the individual has freely chosen to embrace the theistic way. Indeed, for thinkers like Kierkegaard religious faith is not something that can be achieved once and for all. The tensions between belief and unbelief are ever present in the human situation. Again and again the believer must opt for faith, grasping it as a freely choosing human being just when it seems most elusive.
Whether religious existentialism is compatible with Judaism is a complicated question. From the point of view of traditional Judaism, at least, God is a "given." He is ever present and the pattern He has created for humans is there in the Torah whether or not humans choose to accept it. The essence of faith does precede human existence. That is why some Jewish thinkers have seen even religious existentialism as a form of atheism.