"We pray you in Christ's stead, be ye reconciled to God. For he hath made him to be sin for us, who knew no sin; that we might be made the righteousness of God in him." (2 Corinthians 5:20-21)
In the beginning when God created Adam and Eve and placed them in the "very good" creation (Genesis 1:31), He gave them one rule to follow (2:17). They were created to live forever in fellowship with Him, but disobedience to this rule would bring death.
As Creator, He had both the authority to set the rules over His creation and the penalty for breaking rules, declaring "the wages of sin is death" (Romans 6:23). A holy, sinless, everliving God who abhors sin cannot allow disobedience in His presence, and a just God's nature demands a just punishment. Ever since the events of Genesis 3, God and humankind have been estranged, with the awful barrier of sin separating them. Man has been unable to restore fellowship with God on his own! Sin's penalty must be paid!
The angels were present at the creation of Adam and Eve, at their disobedience, and at God's withdrawal of fellowship with them. The angels have ever since been active on behalf of repentant sinners, but they were unable to bring about any permanent solution.
But finally it happened! God, the Creator who had been rejected by His creation, the righteous judge who demanded the penalty of death for sin, Himself had come to pay that penalty, to redeem by His own death His fallen, rebellious creation, and the angels rejoiced. JDM
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 hosts proclaim, "Christ is born in Bethlehem!"
Hark! the herald angels sing, "Glory to the newborn King!"