A Key by its very function and purpose is something which unlocks what is closed. Opens what is hidden. Reveals what is obscured. A key is the means to possessing what was previously out of the reach of possession.
There is only one key to the written Scriptures and that is Christ. There is only one means of opening every book of the Bible, every story and every verse and that is Christ. Without Christ there is no unity in Scripture. Without Him it loses its purpose. Without Him there would be no life and Spirit in it.
"I come in the volume of the book." This verse in the Psalms is repeated in Hebrews 10:7 and is clearly speaking of Christ. When He came in His conception, birth, life, ministry, death, burial, resurrection and ascension it was "in the volume of the book." He came to fulfil every prophesy, typology and shadow revealed in the Old Testament. When Christ came we hear from His own lips constantly such statements as this: "...that the scriptures might be fulfilled...as the scriptures have said...it is written..." All of this reveals that His whole life was conducted in line with the written word of the Old Testament.
We are told: "The Son of man goeth as it is written of him:" (Mt.26:24). And again He Himself states: "For I say unto you, that this that is written must yet be accomplished in me" (Lk.22:37). His whole life was tied up with the written Word of God. It is impossible to mock, belittle or de-emphasis the place of the written Word in a Believers life without devaluing Christ Himself. To think we are more free from the written Scriptures than Christ is pride.
"in the volume of the book it is written of me." The volume of the book, the full teaching of the written Scriptures in the Old Testament are all about Christ. It is written about Him. He is the subject, He is the substance, He is the lesson.
When Christ speaks of the 'volume of the book' He is specifically talking about the Old Testament scriptures. The Old Testament is all about Christ - every part of it. I was once asked why I preached from the Old Testament. I answered 'Because Christ did, the apostles did, the early churches did and great men and woman of God from every age have done the same.'
"I delight to do thy will, O my God: yea, thy law is within my heart." Two things here. First, Christ delighted "to do" the will of God. What was the will of God? It was clearly revealed in the written Scripture. He delighted, enjoyed and rejoiced to do, obey, fulfil and align His whole life with what was written in Scripture. Secondly, the law of God, the Word of God was in His heart. When the Word of God abides, dwells and is written on the heart as well as hidden in the heart it produces a joy in obedience. The true believer is being conformed to Christ and so delights to follow His example, attitude and joy towards the written Scripture. Anyone who speaks lightly of the written Word knows little of Christ and knows little about the key to understanding the written Scriptures.