"[World-views] do man the great service of freeing him from himself. They relieve him of the problem of his concrete existence, of anxiety about it and responsibility for it. That is, of course, the reason why man desires a so-called world-view. He can turn to it when he is confronted by the riddle of destiny and death. He can dismiss the problem of existence from his mind when his existence becomes shattered and precarious. He need not take the moment of crisis seriously, for he can understand it simply as a special case of a general class, fit it into a context, objectify it, and so find a way out of it.
"But this is the primary falsity and it leads necessarily to mistaking the truth of our own existence, since we are viewing ourselves from the outside as an object of scientific investigation."
Thus, a world-view never affords an adequate description of existence. Similarly, all religious world-views paint a distorted picture of God, since one views God from some vantage point "beyond" God, and then determines the precise relationships between God, humans, and nature. However, such a vantage point does not exist and God cannot be objectified, and all world-views are inherently speculative. Moreover, they are the antithesis of faith, since trust is placed in a particular "man-made" conception of God, instead of in God Himself. Bultmann never tires of saying that faith is not a world-view, and that the God of every theistic Weltbild is an idol.
Needless to say, Bultmann's critique of the world-view has not been widely embraced. Today, Christians and non-Christians alike regard Christianity as a type of world-view that directly competes with the various secular world-views afloat (hence the Culture Wars). Thus, one becomes a Christian by adopting its Weltbild, either through assent to various doctrines, by embracing a particular moral code, or through a specific type of spiritual experience. In such a way, a person can be confident that they are a Christian (and are thereby saved) because they think, act, or feel like a Christian.
Of course, this is the most sneaky type of "works-righteousness", and Bultmann's antagonism towards the world-view is simply an extension of Luther's doctrine of justification into the realm of the intellect. It's his version of the "theology of the cross", which rejects any attempt to deal with God on our terms. He makes this clear in the following excerpt from an early sermon, preached in the dark days of WWI:
"If we want to see God, then the first thing we should say to ourselves is that we may not see him as we have conceived him. We must remind ourselves that he may appear to be wholly other than the picture we have made of him; and we must be prepared to accept his visage even if it terrifies us... Has our old picture of him fallen to pieces? If so, then we must first of all be grateful that we have lost our false conception."
The God who is "Wholly Other" cannot be squeezed into any man-made world-view. But is theology possible without one? Barth thinks not, and this issue underlies all the disagreements between Bultmann and his (many) critics. More on this to follow...