George Eldon Ladd said it well…

The very complexity of biblical teaching about the kingdom of God is one of the reasons why such diverse interpretations have arisen in the history of theology. Isolated verses can be quoted for the most of the interpretations which can be found in our theological literature. The Kingdom is a present reality and yet it is a future blessing. It is an inner spiritual redemptive blessing which can be experienced only by way of the new birth, and yet it will have to do with the government of the nations of the world. The Kingdom is a realm into which men can enter now, and yet it is a realm into which they will enter tomorrow. It is at the same time a gift of God which will be bestowed by God in the future and yet which must be received in the present. Obviously no single explanation can do justice to such  a rich but diverse variety of teaching. – George Eldon Ladd in The Gospel of the Kingdom, p. 18.

See Ladd’s text for Scripture references .