The Old and New Testaments are perfectly harmonious! There is not a line or word of one that contradicts the other, but they are not and do not say the same things.
God takes particular pains to mark the difference; in fact, He writes each in a different tongue—the one, Hebrew, having its groundwork in the family of Abraham after the flesh. The other is in Greek, used when God was sending the gospel to the Gentiles as such. Thus the Greek was just as much a representative of Gentile objects, as the Hebrew found its fitting object in Israel. But, for all that, God shows His mind in both.
The distinctive feature of the Old Testament is His government, while the distinctive feature of the New Testament is His grace. Government and grace are totally distinct. Government is always a dealing with man, whereas grace is the revelation of what God is and does. Consequently, the one invariably supposes judgment, and the other is the full display of mercy and goodness. Both find their meeting point in Christ.
As He is the King, He consequently is the head of the government. As He is the Son of God, full of grace and truth, He consequently is the one channel for all the blessing peculiar to the New Testament. His glory, now that the mighty work of redemption is done, accounts for all our characteristic privileges.
W. Kelly