Isaiah 9:2. The people who walked in darkness have seen a great light.
Matthew (4:16) cites this text with reference to Christs specific sojourn in Capernaum and Galilee not for the purpose of establishing and strengthening faith but only for the purpose of encouragement. The apostles and evangelists are much accustomed to doing this.
People. So then the people of the whole world, Jews as well as Gentiles, were in darkness, that is, in error, unrighteousness, notions, a false understanding of the Law, etc. Light is the Gospel, the gift of the Holy Spirit. It is common for Scriptures to repeat and double the same thing, as is done here, for the purpose of emphasis.
v. 3 Thou hast multiplied the nation, Thou hast not increased its joy. Here is pictured the fruit and power of this light which is propagated from day to day, not satisfied with the corner where the Jews live but spread abroad among the nations throughout the world, a thing that irks the Jews. In Deut. 32:21 we read: “I will provoke them with a foolish nation.” The Jews, who think that they alone should be God’s people, interpret this passage in a distorted way, as if the Gentiles, however numerous, would not come to enjoy God nor rejoice in Him.3 But this is the true meaning: Many nations are received into the kingdom of God; this makes the Jews sick, by this they are provoked to anger, envy, zeal, and a spirit of stupor (cf. Rom. 11:8). This is what it means to lessen their joy” (Luther’s Works, vol. 16, p.97).