Isaiah 9: A light has dawned

For Jews living in the first century, it was common for someone to quote one sentence of scripture in order to invoke the entirety of a passage. It was a rabbi training method for the older Rabbi to say one sentence and expect the student-apprentice rabbis, the disciples, to be able to quote what was immediately before and after that sentence.So when the angel says "For unto you is born this day in the city of David a Savior, who is Christ the Lord" (Luke 2:11)to the shepherds,  any Jew reading or hearing the story would harken back to Isaiah 9

²The people walking in darkness
have seen a great light;
a light has dawned
on those living in the land of darkness.
³ You have enlarged the nation
and increased its joy.
The people have rejoiced before You
as they rejoice at harvest time
and as they rejoice when dividing spoils.
⁴ For You have shattered their oppressive yoke
and the rod on their shoulders,
the staff of their oppressor,
just as You did on the day of Midian.
⁵ For the trampling boot of battle
and the bloodied garments of war
will be burned as fuel for the fire.
⁶ For a child will be born for us,
a son will be given to us,

and the government will be on His shoulders.
He will be named
Wonderful Counselor, Mighty God,
Eternal Father, Prince of Peace.
⁷ The dominion will be vast,
and its prosperity will never end.
He will reign on the throne of David
and over his kingdom,
to establish and sustain it
with justice and righteousness from now on and forever.
The zeal of the LORD of Hosts will accomplish this.

Verse 2 speaks of a hope for people living in darkness. Imagine yourself living in a windowless room your whole life, where the only light came from an old light bulb situated in the middle of the room. Your entire life would be viewed with the aid of dim light, and that alone. But if one day, you were brought out of the room, outside into bright sunlight, there would be a bit of fear and a period of adjustment, but after you'd experienced the light, it would make no sense to go back to living in the darkness. Jesus came to offer this same hope and this same light.