The final Old Testament quotation in Hebrews 1 is from Psalm 110:1.
As Hebrews returns again and again to the psalm's announcement that God's
Son is seated at his right hand, three emerge: (1) Jesus, our priest-king,
is in heaven. The venue in which he now carries on his ministries of
reconciliation and rule is the original, eternal dwelling-place of God, not
a mere “copy” on earth. (2) Jesus is seated. He has completed, once-for-all,
his task to atone for our sins through his death on the cross. Nothing more
needs to be done–ever–to provide the ground for our forgiveness. (3)
Jesus' kingdom is in process, moving toward the consummation of his victory
in the subjugation of his enemies. We live our pilgrim lives between the
comings of Christ, past and future. This waiting period prolongs our
sufferings, but also prolongs the gospel's work of grace, transforming
enemies into God's people who “offer themselves willingly…in holy
garments” (Psalm 110:2-3).