The Purpose of Sonship
God has always desired to fellowship with man, but sin came into the human family and separated God from man. The purpose of Sonship was so that God could provide the best means by which He and man could regain their fellowship, and to give dominion over the earth to man again. This fellowship is the result of the sacrifice of the Savior on Calvary. He could only do this by providing complete redemption for mankind. He is the Lamb that was slain from the foundation of the world, Rev. 13:8. In short, the Sonship is about God and man engaging in a loving and healthy relationship that is void of any hindrances (sin), which would separate each from the other. For that to take place a Mediator (the Son) had to provide reconciliation between God and man. This Mediator was realized in the person of Jesus Christ.
Who Fulfilled the Sonship
Jesus Christ fulfills the Sonship. In Colossians 2:9 we are told that in Jesus dwells all the fullness of the Godhead (all the characteristics of God) bodily, and we (the Church) are complete in Him (Jesus). Paul did say that the Gospel message was a mystery, but then he went on to explain what the mystery was: "And without controversy great is the mystery of godliness: God was manifest in the flesh, justified in the Spirit, seen of angels, preached unto the Gentiles, believed on in the world, received up into glory” (I Timothy 3:16). Only Jesus Christ fulfills these six points. Thus proving the adage, “Jesus Christ of the New Testament is Jehovah of the Old Testament.”