The function of a key is to lock or unlock—or better (before the advent of doorknobs in distinction from locks) to open and close (Isaiah 22:22). The holder of this key, then, determines who enters this abyss and who may exit it. That one of God’s servants holds this key in his hand underscores that the Lord himself controls its door.
1 Then I saw an angel coming down from heaven, holding in his hand the key to the bottomless pit and a great chain.