One of the chapters of God, Christ and the World is a critical appreciation of the thought of Rudolf Bultmann. (The earlier quote on demythologization was taken from the same chapter.)
Bultmann’s project of demythologization was tied to his desire to unearth the essential message – the kerygma – of Christian faith. He reinterpreted the language of the New Testament so that it could be seen to be about effecting a transformation of human existence. In part, this was because he believed that modern science excluded the possibility of divine action in the field of nature; divine action has to do, at least so far as we can know, exclusively with God’s relation to humanity. Drawing on Heidegger’s existentialism, Bultmann interpreted the symbols of the Bible as making it possible for us to move from inauthentic to authentic existence. It is not Jesus the man who provides a pattern for us to emulate, or the Atonement and Resurrection as cosmic events that make possible forgiveness and new life, but the proclamation of the possibility of authentic existence, realized in Christ.
Here Ramsey questions whether Bultmann’s “existentialist” interpretation of Christianity is adequate:
…while the existential element in the New Testament has, as we saw, an important place, so that an existential theology today is able to recapture it, there is also an ontological element in the New Testament as utterly essential for New Testament Christianity. In the experience of salvation existentialism may seem to suffice, for the Saviour is definable as ‘what he means to me’. But in the experience of worship the Christian was and is concerned with One who is. The worshipper forgets his own being and experiences in the adoration of One who is, and the ‘isness’ of deity is there, behind and before, now and for ever. The ‘isness’ of deity–prominent in the Old Testament–is reaffirmed when it is revealed that Jesus shares in it. St John shows that the glory of Jesus which men encountered in his life and death is the glory of deity in eternity. Is it that an exclusively Protestant view of Christianity as the religion of the Word, which misses the deep emphasis of Catholic Christianity upon adoration, causes Bultmann to suppose that an existentialist concept can convey the whole meaning of Christianity? Ontology, ‘isness’, ‘being’ is ineradicable from the Christianity of the New Testament. (pp. 56-7)
In retrospect, what seems to me most dated about existentialism–and existentialist interpretations of Christianity–is its excessive anthropocentrism. At the time, existentialism seemed like it offered an end run around the “scientific” worldview that seemed to preclude divine action in the world and to deny the possibility of human freedom. But the price paid was to erect a wall between humanity and nature which later thought has shown to be untenable. The human-divine encounter occurred only in the depths of the self and its transition from “inauthentic” to “authentic” existence. There was no room left for divine revelation through nature, or for the idea that nature might have its own inherent value or meaning. This dovetails to some extent with Ramsey’s concern that “existentialist” Christiantiy talks about God only as the power of salvation for human beings; this risks turning into an instrumentalist view of God and the death of true worship. A better cosmology and theology would allow that 1. human beings are fundamentally part of nature, 2. that God is revealed through all parts of the created order, and 3. that God’s purposes encompass more than human salvation.
See here for a summary of Bultmann’s thought.