Isaiah 9:2 (ESV)
2 The people who walked in darkness have seen a great light;
those who dwelt in a land of deep darkness, on them has light shone.
Darkness and Light are powerful images of sin, death, defeat on the one side and life, goodness and victory on the other. John 1 takes up this theme of Jesus, “The Word”, being both light and life.
This Isaiah reference is applied in Matthew, not to his birth, but to the outset of his ministry. It is thus a reference to his prophetic teaching ministry. Jesus words are a light in the darkness:
Matthew 4:12–17 (ESV)
Jesus Begins His Ministry
12 Now when he heard that John had been arrested, he withdrew into Galilee. 13 And leaving Nazareth he went and lived in Capernaum by the sea, in the territory of Zebulun and Naphtali, 14 so that what was spoken by the prophet Isaiah might be fulfilled:
15 “The land of Zebulun and the land of Naphtali, the way of the sea, beyond the Jordan, Galilee of the Gentiles—
16 the people dwelling in darkness have seen a great light,
and for those dwelling in the region and shadow of death, on them a light has dawned.”
17 From that time Jesus began to preach, saying, “Repent, for the kingdom of heaven is at hand.”
This is a promise for those “in the dark.”