There is a lot of debate and speculation as to what the Key of David is. Some believe it is the true lineage of Jews, which is foolish. It is written in Revelation 3:7 that the Church of Philadelphia has this key to a door that no man can shut or open but Christ. How can the Church of Philadelphia have a key to a lineage? It is this verse that partially gives explanation to the answer of this mystery.
The Key of David is simply the promise God gave to David that through his line would come the Saviour, who would bring an everlasting kingdom. The Church of Philadelphia is only the elect of the last generation. All churches written of in Revelation chapters two and three have to do with churches of the latter days. The Church of Philadelphia, as is written in Romans 8:30, have already been judged innocent due to their faithfulness shown in the previous age. They have this key because God has already judged them. They have not denied His name. No man can take away this key from them; no, not Satan himself. No man can enter the everlasting kingdom unless Jesus Christ opens it up to them, all depending on what they do. No man can shut up the kingdom on someone Christ has opened it up to. This is the Key of David.