'Key' in the Bible

"Alas for you expounders of the Law! for you have taken away the key of knowledge: you yourselves have not entered, and those who wanted to enter you have hindered."

"To the minister of the Church at Philadelphia write as follows: "'This is what the holy One and the true says--He who has the key of David--He who opens and no one shall shut, and shuts and no one shall open.

The fifth angel blew his trumpet; and I saw a Star which had fallen from Heaven to the earth; and to him was given the key of the depths of the bottomless pit,

Then I saw an angel coming down from Heaven, having the key of the bottomless pit, and upon his arm he carried a great chain.