Matt. IV. 12.
“Now when Jesus had heard that John was delivered up, He departed into Galilee.”
1. Wherefore does He depart? Again instructing us not to go to meet temptations, but to give place and withdraw ourselves. For it is no reproach, the not casting one's self into danger, but the failing to stand manfully when fallen into it. To teach us this accordingly, and to soothe the envy of the Jews, He retires to Capernaum; at once fulfilling the prophecy, and making haste to catch the teachers of the world: for they, as you know, were abiding there, following their craft.
But mark, I pray you, how in every case when He is about to depart unto the Gentiles, He has the occasion given Him by Jews. For so in this instance, by plotting against His forerunner, and casting him into prison, they thrust out Christ into the Galilee of the Gentiles. For to show that He neither speaks of the Jewish nation by a part of it, nor signifies obscurely all the tribes; mark how the Prophet distinguishes that place, saying “The land of Nephthalim, by the way of the sea, beyond Jordan, Galilee of the Gentiles, the people which sat in darkness, saw great light:” by darkness here not meaning that which is sensible, but men's errors and ungodliness. Wherefore he also added, “They which sat in the region and shadow of death, to them light is sprung up.” For that you might learn that neither the light nor the darkness which he speaks of are sensible, in discoursing of the light, he called it not merely light, but “a great light” which elsewhere he expresses by the word, True: and in describing the darkness, he termed it, “a shadow of death.”
Then implying that they did not of themselves seek and find, but that God showed Himself to them from above, he says 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. For as persons not even knowing where to put a step forward, so they sat, overtaken by the darkness, not being able so much as to stand any more.
Source: Homilies on the Gospel of St. Matthew (New Advent)