He Recognises That In Order For A Man To Express His Gratitude to God Religious Observances Are Not Enough, And That What God Requires Of Him Is Total Obedience to His Will, Something To Which He Gladly Accedes (Psalms 40:6-8).
‘Sacrifice and offering you have no delight in,
My ears have you provided (literally ‘dug for me'),
Burnt-offering and sin-offering you have not required.
Then said I, Lo, I am come, in the roll of the book it is written of me,
I delight to do your will, O my God,
Yes, your law is within my heart.'
He affirms that he has used his God-provided ears (the ears that God has dug for him) in order to listen to what God has to say, and has recognised that God requires not simply religious observance, but an obedient heart. He had no doubt heard of the words of Samuel to Saul, which would have echoed throughout the land. ‘Has YHWH as great delight in burnt offerings and sacrifices as in obeying the voice of YHWH. Behold to obey is better than sacrifice, and to listen than the fat of rams' (1 Samuel 15:22). And he wants YHWH to know that he will use the ears that He has given him in order to listen.
The whole range of offerings and sacrifices are in mind. Both the blood sacrifices and the meal offerings, and including those most important of offerings, the wholly offered burnt offerings and the sin offerings. All are required by YHWH for the purposes of atonement, but they are not sufficient in themselves. What is more important than all is an obedient heart (compare Isaiah 1:11-18).
And he wants YHWH to know that his obedience is such that the guarantee of it is in writing, for it is written in a book, ‘Lo I am come, I delight to do your will O my God, yes, your Instruction is in my heart'.
The reference may be to his own private record of his own dedication of himself, including some of his psalms, or to the Book of Deuteronomy, or indeed to the whole of the Instruction of Moses (the Pentateuch). But the fact that it is recorded in writing is seen as giving it more force. (For ‘roll of a book' compare Jeremiah 36:2; Jeremiah 36:4; Ezekiel 2:9. In both cases what was written was of immense import).
What is written, however is more important than where it is written. And what is written is that he will come to God, and will with great delight obey Him fully from the heart. This is what is required of all men, to do the will of God (compare Psalms 19:7-14), and have His Instruction written in their heart (compare Jeremiah 31:33). And when the words are applied to our Lord Jesus Christ, great David's greater son, the book is the Scriptures, and the obedience is according to the eternal will of God, but carried out through suffering by our Lord Himself in order that He might be a perfect and complete sacrifice (Hebrews 10:5-14).