Rev 5:5, Isa 22:22, Isa 9:7, Jer 23:5, Rev 1:18

In the Old Testament, it was prophesied that God would give the key of David to Christ, whom he would send. It was also foretold that the Christ would rule the world for ever. (Isa 22:22, Isa 9:7)

Isa 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.

Isa 9:7 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.

In the Old Testament, it was foretold that the Christ would come as a descendant of David. (Jeremiah 23:5)

Jeremiah 23:5 Behold, the days come, saith the Lord, that I will raise unto David a righteous Branch, and a King shall reign and prosper, and shall execute judgment and justice in the earth.

The key of David is the key that can open the gates of heaven. Jesus is the Christ with the key of David. (Rev 1:18, Rev 3:7, Rev 5:5)

Rev 1:18 I am he that liveth, and was dead; and, behold, I am alive for evermore, Amen; and have the keys of hell and of death.

Rev 3:7 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;

Rev 5:5 And one of the elders saith unto me, Weep not: behold, the Lion of the tribe of Judah, the Root of David, hath prevailed to open the book, and to loose the seven seals thereof.