Westminster Larger Catechism
Luke 2:8–21 (CSB) In the same region, shepherds were staying out in the fields and keeping watch at night over their flock. Then an angel of the Lord stood before them, and the glory of the Lord shone around them, and they were terrified. But the angel said to them, “Don’t be afraid, for look, I proclaim to you good news of great joy that will be for all the people: Today in the city of David a Savior was born for you, who is the Messiah, the Lord. This will be the sign for you: You will find a baby wrapped tightly in cloth and lying in a manger.”
Suddenly there was a multitude of the heavenly host with the angel, praising God and saying:
Glory to God in the highest heaven,
and peace on earth to people he favors!
When the angels had left them and returned to heaven, the shepherds said to one another, “Let’s go straight to Bethlehem and see what has happened, which the Lord has made known to us.” They hurried off and found both Mary and Joseph, and the baby who was lying in the manger. After seeing them, they reported the message they were told about this child, and all who heard it were amazed at what the shepherds said to them. But Mary was treasuring up all these things in her heart and meditating on them. The shepherds returned, glorifying and praising God for all the things they had seen and heard, which were just as they had been told. When the eight days were completed for his circumcision, he was named Jesus—the name given by the angel before he was conceived.
Consider all of Old Testament, why were these shepherds so joyful in this little baby boy being born?