What Does the Bible Say About the Kingdom of God?

Many kingdoms come and go…but only one Kingdom remains forever. Yet many Christians seem confused. Does Jesus presently reign — or will He only start when he returns to Jerusalem? If He is reigning, is His kingdom on earth? And if so, where exactly do we see the government upon His shoulder, and is it increasing without end? What is the gospel actually doing…what will it achieve…and how important is it to get this right?