Commentary: "In his sovereign grace God did the wholly unexpected. Not mainly to the Jerusalem aristocracy, but especially to the despised, sorely afflicted, and largely ignorant masses of Galilee, a mixed Gentile-Jewish population, did he send his Son. It was in and around Galilee that Jesus spent most of his incarnate life on earth. It was here that he grew up; here also that he subsequently traveled from village to village on his errands of mercy, imparting comfort and healing, and above all else seeking to save the lost. It was here that he walked the shores and addressed the crowds. It was in this general region that he gathered around him a band of disciples. It was from this northern portion of Palestine that his words of life and beauty, of admonition and consolation, were carried far and wide and from father to son.

In all probability the five items mentioned here in verse 15 refer to five different sections of Greater Galilee. The land of Zebulun was west of the Sea of Galilee, and was bounded on the north by the land of Naphtali. The region toward the sea was to the west of these, and extended from north to south along the Mediterranean. Beyond the Jordan indicates the territory east of the Jordan. The region which because of the strong pagan element in its population is called Galilee of the Gentiles (Galîl in the Old Testament) was the northernmost stretch of what used to be called Naphtali. One of its chief cities during Old Testament times was Kedesh (Josh. 20:7; 21:32). The name of Galîl (Isa. 9:1) was changed to Galilee, and thus altered, became the designation of the entire large province ruled by Herod Antipas.

...The entire population of this quintuple northern part of Palestine is described as “the people sitting in darkness,” and as “those sitting in the land of death’s shadow.” For centuries those living in this large territory had been exposed to political and military aggression from the north (Syria, Assyria, etc.) and to the corrosive moral and religious influences of a pagan environment. They had been overrun and imperiled on a scale much larger than had been the case with respect to the people of Jerusalem and surroundings (see II Kings 15:29; Isa. 8:4). No doubt to many of the inhabitants of Galilee the words recorded in II Kings 17:33 with specific reference to the Samaritans would also be applicable: “They feared Jehovah, and served their own gods.”

Sitting in darkness and in the land of the shadow of death indicates a condition of danger, fear, and hopelessness, a pining away, with no human help in sight. In Scripture the designation darkness, when used figuratively, refers to one or more of the following features: delusion (blindness of mind and heart; cf. II Cor. 4:4, 6; Eph. 4:18); depravity (Acts 26:18); and despondency (Isa. 9:2; see its context, verse 3). Though all three qualities are probably in the picture here, yet the emphasis may well be on the last of the three (despondency, hopelessness), as has already been explained.
The antonym of darkness is light, which, accordingly, refers to genuine learning (the true knowledge of God, Ps. 36:9), life to the glory of God (Eph. 4:15, 24; 5:14), and laughter (gladness, Ps. 97:11). All three may well be included, but here too the emphasis is perhaps on the last of the three.

The real meaning of the quotation, accordingly, is this, that Jesus Christ, by his presence, words, deeds of mercy and power, would fill the hearts of all his Galilean followers with the joy of salvation. No longer would they be pining away in gloom and despair. When Jesus comes into Galilee and begins his great ministry there, the words of a popular hymn go into effect,

The whole world was lost in the darkness of sin,
The light of the world is Jesus.

Matthew concludes this paragraph by stating 17. From that time Jesus began to preach and to say, Be converted, for the kingdom of heaven is at hand. The following points should be noted:

a. In essence, though not in detail, Christ’s message is the same as that of John the Baptist, witness the fact that in 3:2 the herald’s proclamation was summarized in identical words. It is unnecessary therefore to repeat the explanation that has already been given; but see also on 4:23.

b. In connection with the preceding context (4:13–16) the meaning here in verse 17 is that Jesus now begins to bring this gospel of the kingdom into regions which the Baptist had not penetrated to any great extent. The good news is beginning to be spread over a wider territory. The demand that men be converted resounds in regions where it had not been heard before.

c. The fact that the coming of Christ has indeed brought about a tremendous change on earth, so that millions of people have been translated from the realm of darkness into the kingdom of light, shows that the proclamation, “the kingdom of heaven is at hand” was fully justified.

d. Not immediately or all at once was this message proclaimed throughout the world. From the beginning its spread was intended to be progressive: it was to reach first the Jew (10:5, 6), then also, step by step, all the nations (24:14; 28:19; Acts 13:46; Rom. 1:16). It is not surprising, therefore, that the announcement “the kingdom of heaven is at hand,” found first on the lips of the Baptist, then confirmed by Jesus, is at Christ’s command repeated by the disciples (Matt. 10:7), with the intention that it shall at last reach the entire world: every nation. Then shall the end come."

- William Hendriksen, Vol. 9: New Testament commentary : Exposition of the Gospel According to Matthew., Grand Rapids: Baker Book House.