from our beloved St. John Chrysostom, from his homily on Hebrews 9:15ff:
So then also here: The Son became Mediator between the Father and us. The Father willed not to leave us this inheritance, but was angry against us, and was displeased [with us] as being estranged [from Him]; He accordingly became Mediator between us and Him, and prevailed with Him.
And what then? How did He become Mediator? He brought words from [Him] and brought [them to us], conveying over what came from the Father to us, and adding His own death thereto. We had offended: we ought to have died: He died for us and made us worthy of the Testament.