In Latin, "key" is "clavis":
"O Key of David, and scepter of the house of Israel: you open and no one shuts; you shut and no one opens. Come and lead forth from his prison the captive sitting in darkness and in the shadow of death."
Each of the "O" Antiphons begins with a title given to the Messiah. The prayers then state a belief in a certain aspect of God and conclude with a plea for salvation.
But, can our human brains ever fully comprehend and describe God?
My friend, Heather, addressed this question a few days ago in a post at her blog Word Play:
"I think it is a failing of human language to ever be able to express God in fullness."