Jesus Christ is the light of all nations

“The people who walked in darkness have seen a great light;
Upon those who lived in a land of gloom a light has shone.”  (Isaiah 9:1)

The great light is Jesus.  He is the Light of all nations.
I like the exposition of John Gills on Isaiah 9:2:-

“they have seen a great light;
Christ himself, who conversed among the Galilaeans, preached unto them, and caused the light of his glorious Gospel to shine into many of their hearts; by which their darkness was removed, so that they not only saw Christ, this great light, with their bodily eyes, but with the eyes of their understanding; who may be called the "light", because he is the author and giver of all light, even of nature, grace, and glory; and a "great" one, because he is the sun, the greatest light, the sun of righteousness, the light of the world, both of Jews and Gentiles; he is the true light, in distinction from all typical ones, and in opposition to all false ones, and who in his person is God over all.”

Matthew Henry has equally good commentary on the above scripture:

“When the gospel comes to any place, to any soul, light comes, a great light, a shining light, which will shine more and more. It should be welcome to us, as light is to those that sit in darkness, and we should readily entertain it, both because if is of such sovereign use to us and because it brings its own evidence with it. Truly this light is sweet.”