“The land of Zebulun and the land of Naphtali,
Toward the sea, beyond the Jordan, Galilee of the Gentiles,
The people that sat in darkness saw a great light,
And to those who sat in the region and shadow of death,
To them did light spring up.
Matthew's main emphasis in the use of this quotation is to indicate that Jesus has commenced His new ministry in the very place where God said it would take place, and then to bring out the wonder of that ministry. To Matthew it helps to explain why God has begun here. Originally the idea in Isaiah was that these were the furthest outposts of Palestine which were ever the first to be subjected to invading forces, and the point was that with the coming of the child who would be born to be king those fears would disappear, so that where there was darkness and death there would now be light.
Thus now that the Child has been born and the Son has been given, the people who have been in darkness, will now experience a great light, as Isaiah had said. Light and life will come to those who sit in darkness and death, and to such an extent that they too, having received that light, must themselves let it shine out to men (Matthew 5:16). Jesus' ministry is to be a ministry of light (compare Matthew 5:45, ‘God causes His sun to rise on the evil and on the good'; Matthew 6:22, ‘if your eye is single your whole body will be full of light'; Matthew 17:2, ‘He was transfigured before them, and His face shone like the sun, and His clothing became white as light'; Matthew 24:17, ‘for as the lightning comes forth from the east, and is seen even to the west, so shall the coming of the Son of Man be'). For those who respond to Him are to become the light of the world (Matthew 5:14), a light revealed by the purity and goodness of their lives, which shines permanently because they are truly His (Matthew 25:4). It is thus in order to receive this light that men must open their eyes, for the alternative will be pitch darkness (Matthew 6:22-23). This makes clear that these words are very closely associated with the message in the Sermon on the Mount through which the light is revealed as shining. But note that those to whom He is speaking in the Sermon on the Mount are mainly those who have already received this light (although some of them may be responding hypocritically - Matthew 7:13-27). Their eyes have already been opened and they have become disciples.
‘Toward the sea, beyond the Jordan' may be intended to suggest the Great Sea (the Mediterranean) and the borders of the Jordan, illustrating the width of Jesus' ministry. Or Matthew's idea may be to relate ‘the Sea' to the Sea of Galilee. ‘Beyond Jordan' can refer to both sides of the Jordan for it was a popular name for the land around the Jordan. But these place names and ‘Galilee of the Gentiles' are mainly cited because they were contained in the quotation, which is probably taken from a Hebrew text of Isaiah of a type predating the LXX. It is Naphtali and Zebulun that Matthew mainly draws attention to. On the other hand we may certainly gather from all this a further implication (compare Matthew 2:1-2) that the Gentiles are at some stage to be involved in the coming of the light, for as well as mentioning ‘Galilee of the nations' (Isaiah 9:2) Isaiah had also pointed out that the Servant of the Lord would be a light to the Gentiles (Isaiah 42:6; Isaiah 49:6). And the very fact that He preaches in this very mixed region reveals the magnitude of His thinking. Nevertheless it will be made clear that there are lost sheep of the house of Israel out there (Matthew 10:6), and that they have the first claim on His attention, before He can reach out to the Gentiles (Matthew 15:24; Matthew 15:27).