John Bright in The Kingdom of God:
"The gospel according to Mark begins the story of Jesus' ministry with these significant words: "Jesus came into Galilee, preaching the gospel of God, and saying, "The time is fulfilled, and the Kingdom of God is at hand, repent and believe in the gospel" (1:14-15). Mark thus makes it plain that the burden of Jesus' preaching was to announce the Kingdom of God; that was the central thing with which he was concerned. A reading of the teachings of Jesus as they are found in the gospels only serves to bear this statement out. Everywhere the Kingdom of God was on his lips, and it is always a matter of desperate importance.