Andreas of Caesarea — on Rev 3:7 (COMMENTARY ON THE APOCALYPSE 3:7)

“His kingdom is called "the key of David," for this is a symbol of authority. And, moreover, the Holy Spirit is the key of the book of the Psalms and of every prophecy, for through him the treasures of knowledge are opened. He received the first key according to his humanity, but the second key he possesses according to his deity, which has no beginning. Since in some of the copies there is written "hades" instead of "David," through the image of the "key of hades" the authority of life and death is ascribed to Christ.”