'Key' in the Bible

"And to the angel of the church in Philadelphia write: "This is what the holy one, the true one, the one who has the key of David, the one who opens and no one can shut, and who shuts and no one can open, says:

And the fifth angel blew the trumpet, and I saw a star that had fallen from heaven to the earth, and the key of the shaft of the abyss was given to him.

And I saw an angel descending from heaven, holding the key of the abyss and a great chain in his hand.