"But their minds were hardened. For to this day, when they read the old covenant, that same veil remains unlifted, because only through Christ is it taken away."
In a certain sense, you will never fully understand the New Testament unless you understand the Old Testament. The Scriptures of the Old Testament find their fulfillment in the new. Each and every text in the Old Testament is a theological statement about God, whether it be Father, Son, or Holy Spirit. However, you will also never fully understand the Old Testament without the New Testament.
Paul says in this verse that without Christ, there is a veil over the "old covenant", meaning the promises made to God's people Israel (and the entire world through them). There is truth beyond the original truth and beyond the historical contexts. Actually, it would be a mistake to read the Old Testament as simply an accounting of the history of the Jewish people. The Jews did not so painstaking preserve the Scriptures over thousands of years just to have a record of their history. Rather, what they recorded were God's self revelation through His interactions with his people. Each and every text is a theological statement, not a historical synopsis. And without the Spirit of Christ, no one will understand the greater overarching truth of God because Christ is the full revelation of an invisible Father.
So then, just as every text reveals something about Christ, Christ reveals everything about all texts.