Hebrews 10:5-10 (HCSB)
Therefore, as He was coming into the world, He said:
You did not want sacrifice and offering,
    but You prepared a body for Me.
You did not delight
    in whole burnt offerings and sin offerings.
Then I said, “See—
    it is written about Me
    in the volume of the scroll—
    I have come to do Your will, God!”
After He says above, You did not want or delight in sacrifices and offerings, whole burnt offerings and sin offerings (which are offered according to the law ), He then says, See, I have come to do Your will.  He takes away the first to establish the second. By this will of God, we have been sanctified through the offering of the body of Jesus Christ once and for all.

The writer of Hebrews turns to Psalm 40:6-8 to prove his thesis of the superiority of the New Covenant over the Old. This Psalm is a song of praise by David for God’s faithfulness to him over the years, and for the many deliverances that He had provided.

David’s point is that a relationship with God is about far more than simply bringing the required sacrifices at the proper time. Instead, it is about a real relationship, a devotion of one’s whole life, body and soul, to God and to His service.

The writer applies the completer fulfillment of this Psalm to Jesus’ life and ministry. Although Jesus dotted every I and crossed every T of the legal requirements of the law (the real requirements, not those invented by the interpretations of the legal scholars), He never depended on those thrice-yearly sacrifices to be the basis for His relationship or standing with God. Obedience to those requirements was the outgrowth of the relationship, not the source of it.

Jesus’ relationship was established and maintained by a whole-life devotion to the Father, a whole-life commitment to immediately do His will as soon as He knew what that will was. And it was that will, not just a willingness, but a firm commitment do doing that, that was the basis for His sacrificial death, and thus for our own holiness and righteousness before God.

Father, this makes all the sense in the world. We no longer live under a covenant of continual sacrifices, but under a covenant of personal relationship, established and enabled by the one perfect sacrifice of Jesus, and empowered by the indwelling Holy Spirit. But to live in that covenant requires the same whole-life commitment that Jesus demonstrated for us, not the half-hearted commitment that the Jewish Christians were in danger of walking into by returning to the works-based righteousness of the Old Covenant. Lord, inspire in me today exactly that whole-hearted, whole-life-focused devotion to You, so that I can walk with You every step. Amen.