Easton's Bible Dictionary
Kingdom of God
(Matt. 6:33; Mark 1:14, 15; Luke 4:43) ="kingdom of Christ" (Matt. 13:41; 20:21) ="kingdom of Christ and of God" (Eph. 5:5) ="kingdom of David" (Mark 11:10) ="the kingdom" (Matt. 8:12; 13:19) ="kingdom of heaven" (Matt. 3:2; 4:17; 13:41), all denote the same thing under different aspects, viz.

1) Christ's mediatorial authority, or his rule on the earth; (2) the blessings and advantages of all kinds that flow from this rule; (3) the subjects of this kingdom taken collectively, or the Church