I came across this excellent quote today while doing some research in Romans 12:1. Barth has the following to say about preaching and the mercy of God:
For the voice of the preacher, even though it be pitched in the key of absolute truth, wobbles from note to note, is raucous, croaking, and utterly unimpressive. It is, in fact, the cry of a Titan. And, whether the preacher himself be good or bad, he simply bears witness to the judgment which hangs over all Titanism. Human exhortation, therefore, is justified only when it is seen to be void of human justification; that is to say, when it is grounded upon the mercies of God.