“…I would argue that the Old Testament functions within Christian scripture as a witness to Jesus Christ precisely in its pre-Christian form. The task of Old Testament theology is, therefore, not to Christianize the Old Testament by identifying it with the New Testament witness, but to hear its own theological testimony to the God of Israel whom the church confesses also to worship. Although Christians confess that God who revealed himself to Israel is the God and Father of Jesus Christ, it is still necessary to hear Israel’s witness in order to understand who the Father of Jesus Christ is. The coming of Jesus does not remove the function of the divine disclosure in the old covenant” (Brevard Childs, Old Testament Theology in a Canonical Context, p. 9).
The Old Testament is exactly that, an Older Testament. God really did reveal himself to Israel (Heb. 1:1), and as Christians we should seek to understand that witness, for it too glorifies Christ, the Messiah (Acts 18:28).
In the past (in history), the Lord spoke through the prophets, he really did, and we should endeavor to understand the testimony of the prophets. And now the Lord speaks through Jesus Christ, he really does, and we should endeavor to understand the testimony of Jesus Christ. However, to affirm that Christ is the heart of the Scriptures does not mean that we should collapse the witness of prophets into the witness of Christ the Prophet. As Childs states, Jesus did not remove the function of the divine disclosure of the OT.
In the NT, Christ tells us that he fulfils the witness of the prophets. To fulfil does not mean to obliterate (Mat 5:17). And here is the rub: If you separate the OT from the NT or collapse the OT into the NT, then you are not taking the historical fact that God spoke to the prophets seriously, and when you do this you are attacking their witness. And since that very witness corresponds by figure to Heavenly events (Heb 8:5), in reality what you are doing is attacking Jesus Christ. (Leithart makes this very point: Knowledge of the Scriptures equips us to live in the world as it truly is, and I would go on to clarify that this knowledge includes knowledge of the Older Testament. So, knowledge of the Scriptures is knowledge of Christ; and that knowledge, which is grace from God, allows us to live in the world as it truly is. Christ truly is King, and when we do not attack him, that is, when we submit to him and bow our knees to him, then we are actually living in the world as it truly is. And not only living, but through prayer, the sacraments, worship, etc., we are also meaningfully participating in the world as it truly is.)
It is Easter. Please, do not be like the pagans and the hypocrites; please do not attack Christ. If you separate the OT and NT, then you are attacking Christ. If you collapse the OT into the NT, you are attacking Christ. You are not, however, attacking Christ if you hold the OT and NT in union, for their unity is in Christ.