The mutual relationship can be understood by considering a pianist. Few things impress me more than watching a masterful pianist play a seemingly difficult piece of music. At one time their fingers, hands, arms, eyes, feet, and ears all seem to be doing something different, going in different directions, and working independently of one another, yet from that comes one beautiful sound of perfect harmony. Amazing! How? How do all of these parts work separately, accomplishing such grandeur? What makes such complexity possible? The fact is each member is subject to one mind. A.T. Pierson says it this way regarding the Old and New Testaments, “or again, they are like the human body bound together by joints and bands and ligaments; by one brain and heart, one pair of lungs, one system of respiration, circulation, digestion, sensor and motor nerves, where division is destruction.”
Now, that one relating factor manifests itself in many ways between the two Testaments. There are many scribes, distant and unconnected, divided by time and geography, and yet one perfectly harmonizing account is produced. There are various accounts, and yet one marvelous theme! From Genesis to Revelation the story is the reconciliation of man to God, which encompasses many themes, and there is one center piece to it all; the Son of God and the Son of man Jesus of Nazareth, the Christ! God the Father had a plan, a thought, from before the foundations of the earth, and His ways are not our ways, and so when He began the implementation of that thought the entire world was baffled. His son on the cross was the answer. The Old Testament paved the way, and the New Testament represents Jesus walking on the way that was paved.
This unity does not mean that God in addressing specific people at specific times had no real message of relevance for them. Indeed He did. That just testifies to the genius of God that His truth has and maintains timeless relevance. The Book is one, and the Story is one, yet it was unfolded over thousands of years. Regarding this Pierson says, “Unity does not exclude duality. This book is in two principal parts, the Old and New Testaments, not independent of each other but, like the two sides of the human body, organically one; the two hands and feet both by their likeness and unlikeness contribute to mutual efficiency.” So we have the Word of God, one Book, two parts, mutually related.
 Ramm, Bernard. Protestant Biblical Interpretation. Grand Rapids: Baker Books. 1970. p. 1
 Pierson, Arthur, T. Knowing the Scriptures. New York: Global. 1910. p.54
 Pierson, Arthur, T. Knowing the Scriptures. New York: Global. 1910. p.53