It is best to see the unity and the center or thematic principle of the Old Testament in the concept of the kingdom of God. This theme can be seen from the very beginning of Genesis to the concluding words of the prophets. Scripture indicates that God mediates His will on earth through mediators.
At any point in history, beginning in Genesis, God rules His mediatorial kingdom on earth through appointed agents. Adam was the first mediator of God’s kingdom on earth; Messiah will be the final mediator.
God’s purpose for man from the very beginning was that man was destined to rule over creation. Man was to be king of the earth. With the fall of man God has been working to restore man as king of the earth. The ultimate form of man’s rule over the earth will be Messiah’s kingdom.

Enns, Paul P. The Moody Handbook of Theology. Chicago, IL: Moody Press, 1989. Pp. 33-34.