The disciples are to go throughout the towns and villages of Israel, and they are to preach. Christ summarises what they should say, and it is the same message which he preached (Matthew 4:17) – ‘The kingdom of heaven is at hand.
Every minister of the gospel and every witness to the gospel speaks with the authority of the gospel, and the authority of the gospel is the authority of Christ. The authority of the message is not diminished because it passes through fallen and fallible men. If it is preached faithfully, if the preacher deeply respects the message and is careful not to change it in any way, then men and women who hear it may believe it and respond to it, and can claim its promises as if they heard them directly from the mouth of Christ.