Matthew 10:5

We have here the instructions that Christ gave to his disciples, when he gave them their commission. Whether this charge was given them in a continued discourse, or the several articles of it hinted to them at several times, is not material; in this he commanded them. Jacob's blessing his sons, is called his commanding them, and with these commands Christ commanded a blessing. Observe, I. The people to whom he sent them. These ambassadors are directed what places to go to. 1. Not to the Gentiles nor the Samaritans. They must not go into the way of the Gentiles, nor into any road out of the land of Israel, whatever temptations they might have. The Gentiles must not have the gospel brought them, till the Jews have first refused it. As to the Samaritans, who were the posterity of the mongrel people that the king of Assyria planted about Samaria, their country lay between Judea and Galilee, so that they could not avoid going into the way of the Samaritans, but they must not enter into any of their cities. Christ had declined manifesting himself to the Gentiles or Samaritans, and therefore the apostles must not preach to them. If the gospel be hid from any place, Christ thereby hides himself from that place. This restraint was upon them only in their first mission, afterwards they were appointed to go into all the world, and teach all nations. 2. But to the lost sheep of the house of Israel.

— Matthew Henry's Concise Commentary (public domain)

THE TWELVE RECEIVE THEIR INSTRUCTIONS. (Mat. 10:5-42) These twelve Jesus sent forth, and commanded them, saying, Go not into the way of the Gentiles, and into any city of the Samaritans enter ye not--The Samaritans were Gentiles by blood; but being the descendants of those whom the king of Assyria had transported from the East to supply the place of the ten tribes carried captive, they had adopted the religion of the Jews, though with admixtures of their own: and, as the nearest neighbors of the Jews, they occupied a place intermediate between them and the Gentiles. Accordingly, when this prohibition was to be taken off, on the effusion of the Spirit at Pentecost, the apostles were told that they should be Christ's witnesses first "in Jerusalem, and in all Judea," then "in Samaria," and lastly, "unto the uttermost part of the earth" (Act 1:8).

— Jamieson-Fausset-Brown Commentary (public domain)