In the Old Testament, God is eternally king; in a more limited sense, he is king over Israel in so far as, within the covenant bond, Israel acknowledges his royal authority; and in a third sense, his reign belongs to the eschatological future. How often have we been told that it is only in this third sense that the New Testament speaks of the kingdom? Even then there are further restrictions to be made. Is the kingdom present or future? Is it a reign or a realm? Is it a state of mind or a state of affairs? The very fact that these questions can be asked at all should be sufficient indication that it is all these things and more besides. The kingdom of God in the teaching of Jesus is a many-sided concept, and only the context can tell us which aspect of it was uppermost in his mind at any one time. In particular, the classification of the kingdom as an eschatological concept does not sever its connection with history and politics or diminish the unique role which Israel was expected to play in the realization of God’s world-wide sovereignty.
George B. Caird, ‘Eschatology and Politics: Some Misconceptions,’ in Johnston R. McKay and James F. Miller (eds.), Biblical Studies: Essays in Honour of William Barclay (St James Place, London: Collins, 1976): 81