[Paul] asks: "How then shall they call on him in whom they have not believed? and how shall they believe in him of whom they have not heard? and how shall they hear without a preacher (χωρὶς κηρύσσοντος)? and how shall they (the subject of the sentence has obviously changed) preach, except they be sent?" When we quote this passage, we must not miss out the last link in the series. For it is on this that all the rest depends. . . .How, then, can it turn in upon itself? How can it be grounded in itself? How can it crowd out the kerygma [self-proclamation] of Jesus? How can it make this superfluous? It is a call to hearing, faith and confession, news of an accomplished salvation, to the extent that it derives from His sending, consists in the execution of His commission, and therefore, dependent absolutely on His sending and commission, takes up and reproduces the kerygma of Jesus. The power of their action lies in this relationship. It is because of this relationship that there can be said of them and their work: "He that heareth you heareth me" ( Lk. 1016). . . .Without this relationship it would be nothing. It must say always, in supreme concretion: "We preach not ourselves, but Christ Jesus the Lord; and ourselves (we regard as) your servants for Jesus' sake" ( 2 Cor. 45). . . . There is no place, therefore, for any appeal to the undoubted philosophy, scholarship, eloquence, moral impeccability and personal Christianity of the preacher, or for any notion that there is in his preaching any immanent power or value or salvation, or that the Christian kerygma is a self-sufficient and self-operative hypostasis which is as such the πρῶτον [first] and the ἔσχατον [last]. This notion is one of the most monstrous mythologoumena of all times.Christian preaching is the Word of the cross ( 1 Cor. 118), the Word of reconciliation ( 2 Cor. 519). As such it points beyond itself to the concrete history of Jesus Christ. To use the Johannine term, it is witness to Him. The preacher need not be ashamed, therefore, for as such it is the δύναμις [power] of God to salvation to everyone that believes ( Rom. 116). Christ Himself is both its divine σοφία [wisdom] and its divine δύναμις ( 1 Cor. 124). It is not itself the kingdom of God, the divine seizure of power. It makes known the fact that this has happened. It is the proclamation of Jesus as Lord, the giving of factual information and the summons to an appropriate attitude of repentance and faith. And it is all this as the word of the ambassador who is not himself Christ, but speaks in His stead (ὑπὲρ Χριστοῦ).
(Karl Barth, Church Dogmatics IV/2: The Doctrine of Reconciliation, 207-209)