The "I" in the following passage is the writer of Hebrews which is believed to be Paul. He was quoting David in Psalm 40. There David was speaking of himself, but was also prophesying of Jesus. With that understood the "I" in the following passage is you and I also. If they said it, so should we. So that means, as Christians we come in the volume of the Book that is written of us to do thy will oh GOD.
So what Book is written of us?
Heb 10:7 Then said I, Lo, I come (in the volume of the book it is written of me,) to do thy will, O God.
Ps 40:6 Sacrifice and offering thou didst not desire; mine ears hast thou opened: burnt offering and sin offering hast thou not required.
Ps 40:7 Then said I, Lo, I come: in the volume of the book [it is] written of me,
Ps 40:8 I delight to do thy will, O my God: yea, thy law [is] within my heart. David said it of himself and of Christ.
Ps 40:8 Then Paul said it of himself and Christ in Heb 10, He said, lo I come in the volume of the Book it is written of me, to do thy will oh God. There for we can, For God has said, He will put His Law into our hearts and in our minds. As He has said before in
Deut. 29:1-30:14 A New Covenant I will make with you. Not like the one that was made in Horeb. For the Lord our God will circumcise our hearts and the hearts of thy seed. For the Word, the commandments and statutes contained in the Book of the Law are in our hearts and in our mouths that we do them. That is the word of faith in which we preach. The faith that makes us righteous, justifies and establishes the Law. For as by one man's disobedience many were made sinners, so by the obedience of one shall many be made righteous. They who walk after the Spirit and not after the flesh that the righteousness of the LAW be fulfilled. For they do not live by bread alone, but by every word that proceedeth out of the mouth of God