"The kingdom of God lies at the heart of what Jesus had to say, so it is important to grasp what it means.  As we have seen, many different groups in Palestine were looking for the kingdom of God.  Of course, they interpreted it in many different ways.  But they were all waiting for it -- apart, perhaps, from the Sadducees, who were moderately content with the political and religious compromises that kept them in power.  But everyone else was eagerly longing for the day when God would step in for His beleaguered people, defeat the hated Romans, and bring His rule to bear.  So 'kingdom of God' was a phrase packed with power.  And here was this carpenter claiming that with His arrival it was already at hand.

If you study the phrase 'kingdom of God' in the Gospels, you will find that the primary meaning is God's sovereign rule.  The trouble is that most people are rebels against God's way of doing things and want to paddle their own canoe.  They are still within the realm of God's sovereign rule, of course, but are rebel subjects.  And so the phrase 'kingdom of God' comes to have a secondary meaning: the people who willingly accept God's rule in their lives, seek to live by it and extend it.

Imagine, therefore, the excitement when Jesus followed John's call to radical change by this proclamation that the long-awaited kingdom of God was at hand.  He was proclaiming it.  He was bringing it in.  No wonder crowds materialized from nowhere and gathered around Him.  Surely this would be the time for the decisive defeat of the Romans!  But Jesus had a great surprise for them."
Who is this Jesus, Michael Green, pages 29,30.