In this sixth of the seven letters to the seven churches, Jesus says that he is the holy one and the true one. He also mentions something that he is holding:

Jesus also identifies himself as the one who has the key of David, who opens and no one will shut, who shuts and no one opens. Again, those familiar with the Old Testament would recognize these words, this time a quote from the book of Isaiah: “In that day I will summon my servant, Eliakim son of Hilkiah. I will clothe him with your robe and fasten your sash around him and hand your authority over to him. He will be a father to those who live in Jerusalem and to the house of Judah. I will place on his shoulder the key to the house of David; what he opens no one can shut, and what he shuts no one can open.” (Isaiah 22:20-22) In the case of Eliakim, the key to the house of David would have been a large wooden key which would have been worn like a sash. It denoted authority, like carrying the signet ring of the king. The key would have been used to bar and unbar the palace doors, controlling who had access to the king and who could enter or leave the palace grounds.6 In the case of Jesus, we need to remember that each of the letters refers to chapter 1 of Revelation. In the vision that John had of Jesus, Jesus had keys in his hand, the keys to death and Hades. It seems likely that Jesus is referring to the same keys here; the reference to the key of David is made to emphasize the permanence of Jesus’ actions regarding death and Hades. Now that he has opened that door, no one can shut it again, until Jesus himself decides to do so. Christians can face death without fear, for their Lord holds the key to let them out of the realm of death. Their stay in Hades will be but a temporary one, for they know the One who holds the key. When he opens the door, no one can shut it again. When Christ spoke with Peter about having the keys of the kingdom, he also spoke about the gates of Hades, saying that they would never be able to prevail against his church. Jesus opens the door to the place where the dead are, and once he has opened it, no one can shut it.

(Letters From The Lamb, pp. 135-36)

Jesus was calling his followers to be willing to be sacrificed. He reassures them that he has the key that will free them from death and grant them access to God. It’s a promise that was made to the whole church, that the gates of Hades would not prevail against it. Jesus holds the key. Any door that he opens will remain open. And he has thrown open the door to the realm of death. It cannot hold those that Jesus sets free. (And that’s great news!)