Isaiah 22:22 And the key of the house of David will I lay upon his shoulder; so he shall open, and none shall shut; and he shall shut, and none shall open.
Isaiah 9:6-7 For unto us a child is born, unto us a son is given: and the government shall be upon his shoulder: and his name shall be called Wonderful, Counsellor, The mighty God, The everlasting Father, The Prince of Peace. Of the increase of his government and peace there shall be no end, upon the throne of David, and upon his kingdom, to order it, and to establish it with judgment and with justice from henceforth even for ever. The zeal of the LORD of hosts will perform this.
Revelation 3:7-9 And to the angel of the church in Philadelphia write; These things saith he that is holy, he that is true, he that hath the key of David, he that openeth, and no man shutteth; and shutteth, and no man openeth; I know thy works: behold, I have set before thee an open door, and no man can shut it: for thou hast a little strength, and hast kept my word, and hast not denied my name.
The book of Isaiah, often called the Old Testament Gospel, reveals that a child was to be born and his name called "The Mighty God, and the Everlasting Father". We know that this Child was Yeshua (Jesus) of Nazareth, that He is the unique Son of God, the express image of the invisible God. The throne of David was to be given to Him and He now holds its "key", a symbol of the right and authority of His reign, which will be consummated when He returns to this world and restores the Kingdom to Israel [Acts 1:6-7].
The glorified Yeshua, speaking to the church at Philadelphia, mentions this "key of David", reminding them of His authority, and expressing His approval of their deeds, patience and faithfulness to His word, and His love for them. The Lord's approval comes with a mandate. Knowing that they have but a little strength, notwithstanding, He sets before them an open door that no man can shut. They are to move out, through this "door", having been approved, and now equipped to represent Him.
"Philadelphia" means "brotherly love". Can it be coincidence that this church is encouraged with a reminder that the beloved psalmist/King David's key and authority are now held by Yeshua, who sends them on a mission of love?
Those of us whose patience has been tested, will also receive His approval, His protection, and His mandate, just as the church of Philadelphia did. Since God is love, and love is patient, our preparation of patience will equip us with His power to fulfill His mandate, and to go through His "open door". So doing, we will hasten the coming of His glorious Kingdom. Even so, come Lord Yeshua (Jesus)!