John Chrysostom

Then implying that they did not of themselves seek and find, but that God showed Himself to them from above, he saith to them, "Light is sprung up;" that is, the light of itself sprang up and shone forth: it was not that they first ran to the light. For in truth the condition of men was at the worst before Christ's coming. Since they more than "walked in darkness;" they "sat in darkness;" a kind of sign that they did not even hope to be delivered. …

Homily on the Gospel of Matthew 14 ↗

Theophylact of Ohrid

The "great light" is the Gospel, for the law, too, was a light, but a small one. "The shadow of death" is sin, for sin is the likeness and silhouette of death. Just as death overpowers the body, so too does sin overpower the soul. The light has dawned on us, for we were not seeking it, but it appeared to us as if it were pursuing us.

Commentary on Matthew ↗

Thomas Aquinas

The people who sat in darkness have seen a great light. He says two things: who walk and who sat; for one in the darkness, which is not very dense, and who is not dazed by it, walks, especially when he hopes to find light; but when he is dazed by the darkness, he stands. That is the difference between Jews and Gentiles; because although the Jews were in darkness, they were not entirely overcome by it, because not all of them worshiped idols but hoped that Christ would come. …

Commentary on Matthew ↗