In contrast to Adam and Israel in the wilderness, Jesus said, "My food is to do the will of him who sent me and to accomplish his work" (Jn 4:34). God prepared a body for the eternal Son to be given not only for atonement but for that living obedience for which humanity was created. Even more than his sacrifice of atonement, the positive sacrifice of thanksgiving is God's delight (Heb 10:5-10). As our priestly representative, Jesus fulfills Psalm 40:6-8, which the writer to the Hebrews puts on the lips of Jesus: "Then I said, 'Here I am—it is written about me in the scroll"―I have come to do your will, O God'" (Heb 10:7, my translation, emphasis added)!
Jesus' priesthood does not, therefore, begin at Golgotha, but from eternity to his incarnation, life, and death, all the way to his present intercession in glory. His priestly life is referred to as his active obedience (i.e., actively obeying the entire law), distinguished from his passive obedience (i.e., his suffering at the cross). In short, Christ is our priestly Savior by offering both the lifelong "living sacrifice" of praise and thanksgiving and by offering himself as the guilt sacrifice for our sins. He was not only sinless but righteous, not only a nontransgressor of the law but the joyful fulfiller of all righteousness. His commission was to bring not only forgiveness of sins but also that positive righteousness that God wills for us and his world―and beyond even this, the confirmation in that righteousness, peace, and blessedness which the Tree of Life was the sacramental sign and seal.
The Christian Faith. A Systematic Theology for Pilgrims On the Way. Michael Horton. Zondervan, Grand Rapids, MI. 2011. Page 490.