David was the great king of Israel, whose kingdom God had promised to establish forever (see 2 Samuel 7:13; Psalm 89:27–37). David died, but Jesus was the great Son of David through whom the Lord would fulfill his promise (Matthew 1:1–25). So Christ after his victory on the cross could declare that all authority in heaven and on earth had been given to him (Matthew 28:18; see also Revelation 1:5). A key guarantees ownership of and access to a house (see Isaiah 22:22). Christ claims that he possesses the key that permits access to David, and thus to David’s kingdom. That would include the New Jerusalem (Revelation 3:12), the glorious new city of David.
7 “And to the angel of the church in Philadelphia write: ‘The words of the holy one, the true one, who has the key of David, who opens and no one will shut, who shuts and no one opens.