Then I said, "Behold, I have come; in the scroll of the book it is written of Me: I delight to do Your will, O My God."
The right basis for knowing Christ's suffering is to know and understand not only His suffering but also His heart and will for that suffering. Whoever looks at His suffering without seeing His will and heart in them must be terrified at it rather than rejoice in it. But if we see His heart and will in [His suffering], this produces true comfort, confidence, and joy in Christ. For this reason Psalm 40 [:7-8] praises this will of God and of Christ in suffering: "In the book it is written of Me that I should do Your will, O My God, and I do it gladly." The Epistle to the Hebrews says: "By that will we all have been sanctified" (Hebrews [10:10]). He does not say "through the suffering and blood of Christ" (which is certainly true), but "through the will of God and of Christ," because they both were of one will: to sanctify us through the blood of Christ.
From the Church Postil, sermon for Quinquagesima on Luke 18:31-43 (Luther's Works 76:350-51)