There is one line that is common to both Matthew's and Luke's birth accounts: the words of the angel, "Do not be afraid." Those are words rooted in God’s promise to Mary and Joseph.
These are words rooted in God’s own promise to us. He has matched you to Himself through the birth of His own divine Son born in our human flesh. He has matched you to Himself, a match made in heaven. Rooted in Christ and His promises, those words, “Do not be afraid,” have the power to carry out what they say.