Barth, CD I/2, p. 750-751:

The human impossibility of the Church's proclamation consists simply in the impossibility of the attempt to speak of God....

Of God it is impossible to speak... If we speak of Him, we are no longer speaking of Him. In this matter we cannot do what we want to do and we cannot attain what we should like to attain. This is the iron law under which all Church proclamation without exception stands. That what happens here is frailty is far too weak an expression for the real situation. This is not frailty. This is death. This is not difficulty. It is sheer impossibility. What happens here is not something imperfect. Measured by the standard of what is intended, it is simply nothing.

If there is proclamation, if the attempt does not fail, it is just at the point where success is achieved that it can and will be understood, not as human success, but as a divine victory concealed in human failure, sovereignly availing itself of human failure.