Anticipation mounts as the promised Savior King is announced to a lowly Jewish virgin (through whom he’ll be born): “He shall be great, and shall be called the Son of the most High; and the Lord God shall give unto him the throne of David his father; and he shall reign in the house of Jacob for ever. And of his kingdom there shall be no end.”

O Key of David, and Scepter of the House of Israel, who opens and no man shuts, who shuts and no man opens; Come and bring forth the captive from his prison, he who sits in darkness and in the shadow of death.

Isaiah had prophesied:

• “I will place on his shoulder the key of the house of David; he shall open, and no one shall shut; he shall shut, and no one shall open.” Isaiah 22:22
• “His authority shall grow continually, and there shall be endless peace for the throne of David and his kingdom. He will establish and uphold it with justice and with righteousness from this time onwards and for evermore.” Isaiah 9:7

Also compare Isaiah Isaiah 22:22 and Isaiah 42:7.