On one occasion, Jesus took his disciples to visit the mouth of a large cave which was considered by the Greeks to be one of the entrances to the underworld, the realm of the dead ruled over by the god Hades.
It is here that Jesus gives to Peter the (symbolic) keys to the ‘kingdom’ of heaven. These keys unlock/lock the door to the underworld, wresting control over Hades’ jealously-guarded ‘kingdom’. In other words, this event points to death being transformed into life (the possibility of return to the world from the underworld) and judgement being transformed into love (Jesus’ authority being expressed in setting captives free) and hell being transformed into heaven (the ‘kingdom’ of Hades being overthrown by the ‘kingdom’ of heaven, the souls of the dead set free and the furies bound).
Moreover, these keys, used on earth to unlock/lock the door of hell simultaneously unlock/lock the door of heaven. Heaven is now aligned to earth, or one with earth, with earth being the key (to play on words) location. And so this event also points to heaven being transformed into earth (earth becoming the dwelling place of God and his people).
O Clavis David
O Key of David and sceptre of the House of Israel;
you open and no one can shut;
you shut and no one can open:
Come and lead the prisoners from the prison house,
those who dwell in darkness and the shadow of death.