The Word of God which was given in Jesus Christ is a unique historical fact, and everything Christian is dependent on it; hence everyone who receives this Word, and by it salvation, receives along with it the duty of passing this Word on; just as a man who might have discovered a remedy for cancer which saved himself, would be in duty bound to make this remedy accessible to all. Mission work does not arise from any arrogance in the Christian Church; mission is its cause and its life. The Church exists by mission, just as a fire exists by burning. Where there is no mission, there is no Church; and where there is neither Church nor mission, there is not faith. It is a secondary question whether by that we mean Foreign Missions, or simply the preaching of the Gospel in the home Church. Mission, Gospel preaching, is the spreading out of the fire which Christ as thrown upon the earth. He who does not propagate this fire shows that he is not burning. He who burns propagates the fire. This 'must' is both things – an urge and a command. An urge, because living faith feels God's purpose as its own. 'Woe is unto me if I preach not the gospel,' says Paul. Necessity is laid upon him. But also he ought to preach; with the gift, he receives the obligation. 'Go ye into all the world and preach the gospel.’ Whether Christ's command was uttered just in these words, we do not know exactly. But there can be no doubt that He had sent out His disciples with the strict order to preach the gospel of the Kingdom to all the world. Even if Jesus had not done that, it would still be a divine command for everyone who receives the message; for he knows that the divine remedy must be made accessible to all. The classical expression for this propagating activity is not doctrine by kerygma, i.e., the herald's call. The Herald, the keryx, is a man who in the market-place of a city promulgates the latest decree of the king. He is the living publicity organ of the sovereign's will. The herald makes known what no one could know before what the king has decreed. It is just this that the Apostles mean by kerygma. They brought not only good tidings but new tidings as well.
Emil Brunner, The Word and the World, 108.