Hark the herald angels sing

Sometimes we are so familiar with Christmas carols that we don't pay attention to the good (or sometimes not so good!) stuff in the words. The incarnation - God entering the world as a baby - was such a big deal to the angels and everyone else that heard about it that they were unable to stop themselves worshipping, praising and glorifying God. The words of Hark the herald sing capture some of the reason why.

Hark the herald angels sing
"Glory to the newborn King!
Peace on earth and mercy mild
God and sinners reconciled"

Joyful, all ye nations rise

Join the triumph of the skies

With the angelic host proclaim:

"Christ is born in Bethlehem"
Hark! The herald angels sing

"Glory to the newborn King!"
This baby who was born is the ultimate King, the Christ (not Jesus' surname, but a title - the Messiah, the chosen one, the anointed one) and is here to bring reconciliation between God and sinners (you and me). We make ourselves the king of our own lives and we mess things up. We don't have peace, we don't show mercy, we sin, we need help but are unable to help ourselves. God is stepping in to save us.
Christ by highest heaven adored
Christ the everlasting Lord!
Late in time behold Him come

Offspring of a Virgin's womb

Veiled in flesh the Godhead see

Hail the incarnate Deity
Pleased as man with man to dwell

Jesus, our Emmanuel

Hark! The herald angels sing

"Glory to the newborn King!"
The all powerful, creator of the universe became a human. God has become flesh. We will never fully understand this but it demonstrates God's love for us, that He is not a distant, far off God looking down saying "you got yourself into this mess, let's see you get out of it". Instead, the highest authority, the everlasting Lord came alongside us as a man. Emmanuel - God with us.
Hail the heaven-born Prince of Peace!
Hail the Son of Righteousness!
Light and life to all He brings

Risen with healing in His wings

Mild He lays His glory by

Born that man no more may die

Born to raise the sons of earth

Born to give them second birth

Hark! The herald angels sing

"Glory to the newborn King!"
Because of Jesus birth, death and resurrection, death is conquered. He came so that we could have eternal life. We are not just killing time now until that eternal life starts, because our new, regenerated, eternal life to the full starts now.