Heidegger delivered this lecture on two occasions, in 1927 and 1928. Philosophy, as the “ontological” science of universal being itself, differs absolutely from theology, which is the ontic science of God as a particular entity. Theology is a positive science because it deals with a posited entity, which makes it more like chemistry than philosophy. The positum of Christian theology is Christianness (Christlichkeit), that is, the factual mode of existing of a believing Christian. Christian existence is determined by the history that is set in motion by the cross, the crucified, and Christ on the cross. The task of theology is to seek the Word that is able to make us believe and to safeguard us in our faith. This means that theologies must learn to listen again to the Word of God through a destruction of Christian theology. They must attempt to bring the existential rebirth that comes by faith to conceptual form. Theology is the science of existing in faith, that is, of existing historically as a Christian.
Theology is founded on faith, which does not need philosophy, but theology as the ontic science of faith does. What it means to be a Christian can only be lived and experienced in faith. The cross and sin as existentiell determinations of the ontological structure of guilt can be conceptualized with the help of philosophy. Sin presupposes guilt. Theology can thus receive the direction of its inquiry from theology.
The lecture is also Heidegger’s farewell to theology as a matter of personal concern. He now calls faith the existentiell enemy of philosophy. To each other they appear to be mere foolishness. Heidegger dedicated this lecture to his colleague at Marburg, Rudolf Bultmann.