While Jesus planted seeds in Samaria (John 4:39-40), his time was getting short and it was important to first have a solid reach within the Jews (Matt 10:5-7). The Samaritans having experienced Jesus' presence would make it easier for Jews to reach the Samaritans when the gospel spread (Matt. 28:16-20; Acts 1:8).
When Jesus sent out the Twelve, the time at their disposal was short, and it was necessary to concentrate on the people who had been specially prepared for the message of the kingdom. Even if the Twelve did confine themselves to the “lost sheep of Israel,” they would not have time to cover all of these. This has sometimes been thought to be the point of the cryptic words “you will not have gone through all the towns of Israel, before the Son of man comes” (Mt 10:23 RSV).
Kaiser, W. C., Jr., Davids, P. H., Bruce, F. F., & Brauch, M. T. (1996). Hard sayings of the Bible (p. 375). Downers Grove, IL: InterVarsity.
This prohibition against going among the Gentiles and the Samaritans was for this special tour. They were to give the Jews the first opportunity and not to prejudice the cause at this stage. Later Jesus will order them to go and disciple all the Gentiles (Matt. 28:19).
Robertson, A. T. (1933). Word Pictures in the New Testament (Mt 10:5). Nashville, TN: Broadman Press.
But the great reason for this command is simply this—wise commanders know that they must limit their objectives. They must direct their attack at one chosen point. If they spread their forces here, there and everywhere, they dissipate their strength and invite failure. The smaller their forces, the more limited the immediate objective must be. To attempt to attack on too broad a front is simply to court disaster. Jesus knew that, and his aim was to concentrate his attack on Galilee; for Galilee, as we have seen, was the most open of all parts of Palestine to a new gospel and a new message (cf. the discussion of Matthew 4:12–17). This command of Jesus was a temporary command. He was the wise commander who refused to diffuse and dissipate his forces; he skilfully concentrated his attack on one limited objective in order to achieve an ultimate and universal victory.
Barclay, W. (2001). The Gospel of Matthew (Third Ed., pp. 420–421). Edinburgh: Saint Andrew Press.
It would be the Jewish who would spread the gospel to first the Samaritans and then the Gentiles.
Moreover, it is taught in the prophetic writings of the Old Testament, and nowhere more clearly than in Isaiah 40–55, that when Israel grasps the true knowledge of God, it will be its privilege to share that knowledge with other nations. Nearly thirty years later, Paul, apostle to the Gentiles though he was, lays down the order of gospel presentation as being “first for the Jew, then for the Gentile” (Rom 1:16). This statement of primitive evangelistic policy was evidently founded on Jesus’ own practice.
Kaiser, W. C., Jr., Davids, P. H., Bruce, F. F., & Brauch, M. T. (1996). Hard sayings of the Bible (pp. 375–376). Downers Grove, IL: InterVarsity.
The ban on entering any town of the Samaritans is to be understood in the same way. Samaritans were not Jews, but neither were they Gentiles. Jesus did not share his people’s anti-Samaritan bias (although the evidence for this is supplied by Luke and John, not by Matthew), and after his death and resurrection his message of salvation was effectively presented to Samaritans even before it was presented to Gentiles (Acts 8:5–25).
Kaiser, W. C., Jr., Davids, P. H., Bruce, F. F., & Brauch, M. T. (1996). Hard sayings of the Bible (p. 376). Downers Grove, IL: InterVarsity.
Barclay gives a good answer to those who double Jesus actually said what was in Matt. 10:5.
But there are certain things to be remembered. This saying is so uncharacteristic of Jesus that no one could have invented it; he must have said it, and so there must be some explanation.
Barclay, W. (2001). The Gospel of Matthew (Third Ed., p. 419). Edinburgh: Saint Andrew Press.