A messianic psalm
Psalm 40: I Come to Do Thy Will
Psalm 40 begins with David lifted from a pit and given a new song. He learns that God desires obedience more than sacrifice, so he offers himself, saying he has come to do God's will, with God's law within his heart. The psalm then turns to fresh trouble and asks God to hurry to help.
“Then said I, Lo, I come: in the volume of the book it is written of me,”Psalm 40:7
The psalm itself
Psalm 40 is a psalm of David that joins thanksgiving and petition. It opens with rescue, as God draws him out of a miry pit, sets his feet on rock, and puts a new song in his mouth so that many will trust the LORD. From that rescue David draws a lesson at the heart of the psalm. God does not ultimately delight in burnt offerings and sacrifices, but in a devoted life. So David responds that he has come, with the scroll written of him, to do God's will gladly, having God's instruction within his heart. He proclaims God's righteousness in the assembly. The psalm then shifts to present distress, as troubles surround him and enemies seek his ruin, ending with an urgent plea that the LORD not delay in coming to help His servant.
How the New Testament reads it of Christ
Hebrews 10:5 to 7 quotes the obedience section of Psalm 40 and places it on the lips of Christ entering the world. Following the Greek translation, the text reads a body thou hast prepared me where the Hebrew speaks of opened ears, and the writer builds his argument on it. The point is that animal sacrifices could never take away sins, so Christ came to do God's will by offering His own body once for all. This is a direct quotation applied explicitly to Jesus, though the New Testament uses only the verses about doing God's will, not the psalm's later confession of personal sin, which belongs to David's own voice. It is an honest, selective, and powerful christological reading.
Christ in this psalm
The center of Psalm 40 becomes the center of the gospel. Christ is the one who truly says I come to do Thy will, and whose whole life fulfills the scroll written of Him. Where sacrifices only pointed forward, Jesus offered the one obedience God had always sought, giving His prepared body on the cross. Hebrews reads this as the moment Christ sets aside the old sacrificial system to establish the new. David could sing of desiring obedience over sacrifice, yet he still needed atonement and confessed his own guilt. Jesus alone offers the flawless obedience the psalm describes, and by that single willing offering He makes His people holy. The pit David was lifted from becomes, in Christ, the grave from which resurrection lifts us all.
Questions people ask
Why does Hebrews say a body when Psalm 40 says ears?
Hebrews 10:5 follows the Greek Septuagint, which renders the Hebrew opened ears as a prepared body. Both express the same idea of a life made ready to obey God. Hebrews uses the phrase to point to Christ's body offered once for all, so the wording difference serves the same meaning of wholehearted obedience over sacrifice.
Does the whole of Psalm 40 apply to Jesus?
No, and it is honest to say so. Hebrews 10:5 to 7 applies only the section about coming to do God's will. The psalm later confesses the writer's own sins, which fits David but not the sinless Christ. So the New Testament reads the obedience portion of Christ while leaving David's personal confession in David's own voice.