From: Luke 9:51-56
Some Samaritans Refuse to Receive Jesus
 When the days drew near for Him (Jesus) to be received up, He set His face to go to Jerusalem.  And He sent messengers ahead of Him, who went and entered a village of the Samaritans, to make ready for Him;  but the people would not receive Him, because His face was set toward Jerusalem.  And when His disciples James and John saw it, they said, "Lord, do You want us to bid fire come down from Heaven and consume them?"  But He turned and rebuked them.  And they went on to another village.
51. "When the days drew near for Him to be received up": these words refer to the moment when Jesus will leave this world and ascend into Heaven. Our Lord will say this more explicitly during the Last Supper: "I come from the Father and have come into the world; again, I am leaving the world and going to the Father" (John 16:28). By making His way resolutely to Jerusalem, towards His Cross, Jesus freely complies with His Father's plan for His passion and death to be the route to His resurrection and ascension.
52-53. The Samaritans were hostile towards the Jews. This enmity derived from the fact that the Samaritans were descendants of marriages of Jews with Gentiles who repopulated the region of Samaria at the time of the Assyrian captivity (in the eight century before Christ). There were also religious differences: the Samaritans had mixed the religion of Moses with various superstitious practices, and did not accept the temple of Jerusalem as the only place where sacrifices could properly be offered. They built their own temple on Mount Gerizim, in opposition to Jerusalem (cf. John 4:20); this was why, when they realized Jesus was headed for the Holy City, they refused Him hospitality.
54-56. Jesus corrects His disciples' desire for revenge, because it is out of keeping with the mission of the Messiah, who has come to save men, not destroy them (cf. Luke 19:10; John 12:47). The Apostles are gradually learning that zeal for the things of God should not be bitter or violent.
"The Lord does everything in an admirable way [...]. He acts in this way to teach us that perfect virtue retains no desire for vengeance, and that where there is true charity there is no room for anger--in other words, that weakness should not be treated with harshness but should be helped. Indignation should be very far from holy souls, and desire for vengeance very far from great souls" (St. Ambrose, "Expositio Evangelii Sec. Lucam, in loc.").
An RSV footnote after the word "rebuked" in verse 55 points out that other ancient authorities add "and He said `You do not know what manner of Spirit you are of; for the Son of Man came not to destroy men's lives but to save them'". These words appear in a considerable number of early Greek MSS and other versions and were included in the Clementine Vulgate; but they do not appear in the best and oldest Greek codexes and have not been included in the New Vulgate.
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland.
Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.