O Key of David, and sceptre of the house of Israel,
who opens and no one can shut, shuts and no one can open:
Come and bring the prisoners from the prison house,
who dwell in darkness and the shadow of death.
That Jesus opens is wonderful news; that he shuts seems to me right now to be even better news. Open a door, let the prisoners out - but where will they do, and what will they do? What if the prisoners are sinners? What if they - we - are so accustomed to darkness that they would wander back into the prison without a second thought? A life set free in this way is a shapeless, under-determined, and ultimately desperately dangerous life.
But Jesus also shuts, and no one can open. He bars the way back into darkness. Try as we might, we can't go back to where we were or who we were before. God has come into our world, and that can't be undone.