From St. Cyril of Alexandria’s commentary on Luke 9:51-56:
What, then, was the purpose of this occurrence? He was going up to Jerusalem, as the time of His passion was already drawing near. He was about to endure the contumelies of the Jews; He was about to be set at nought by the scribes and Pharisees; and to suffer those things which they inflicted upon Him when they proceeded to the accomplishment of all violence and wicked audacity. In order, therefore, that they [the disciples] might not be offended when they saw Him suffering, as understanding that He would have them also to be patient, and not to murmur greatly, even though men treat them with contumely, He, so to speak, made the contempt they met with from the Samaritans a preparatory exercise in the matter. They had not received the messengers. It was the duty of the disciples, treading in the footsteps of their Lord, to bear it patiently as becometh saints, and not to say anything of them wrathfully. But they were not yet so disposed; but being seized with too hot indignation, they would have called down fire upon them from heaven, as far as their will went. But Christ rebuked them for so speaking.
See here, I pray, how great is the difference between us and God: for the distance is immeasurable. For He is slow to anger, and long-suffering, and of incomparable gentleness and love to mankind: but we children of earth are quick unto anger, hasty unto impatience, and refuse with indignation to be judged by others when we are found out in committing any wrong act; while we are most ready to find fault with others. And therefore God the Lord of all affirms, saying; “For My thoughts are not as your thoughts, nor your ways as My ways; but as the heaven is far from the earth, so are My ways from your ways, and My thoughts from your thoughts.” Such, then, is He Who is Lord of all: but we, as I said, being readily vexed, and easily led into anger, take sometimes severe and intolerable vengeance upon those who have occasioned us some trifling annoyance: and though commanded to live according to the Gospel, we fall short of the practice commanded by the law. For the law indeed said, “Eye for eye; tooth for tooth; hand for hand:” and commanded that an equal retribution should suffice: but we, as I said, though perhaps we have suffered but a trifling wrong, would retaliate very harshly, not remembering Christ, who said: “The disciple is not greater than his teacher, nor the slave than his master;” Who also, “when He was reviled, reviled not again; when suffering, threatened not; but committed His cause to Him Who judgeth righteously.” As treading this path much-enduring Job also is justly admired: for it is written of him, “What man is like Job, who drinketh wrongs like a draught?” For their benefit, therefore, He rebuked the disciples, gently restraining the sharpness of their wrath, and not permitting them to murmur violently against those who sinned, but persuading them rather to be longsuffering, and to cherish a mind immovable by ought of this.
It benefited them also in another way: they were to be the instructors of the whole world, and to travel through the cities and villages, proclaiming everywhere the good tidings of salvation. Of necessity, therefore, while seeking to fulfil their mission, they must fall in with wicked men, who would reject the divine tidings, and, so to speak, not receive Jesus to lodge with them. Had Christ, therefore, praised them for wishing that fire should come down upon the Samaritans, and that so painful a torment should be inflicted upon them, they would have been similarly disposed in many other instances, and when men disregarded the sacred message, would have pronounced their condemnation, and called down fire upon them from above. And what would have been the result of such conduct? The sufferers would have been innumerable, and no longer would the disciples have been so much physicians of the sick, as torturers rather, and intolerable to men everywhere. For their own good, therefore, they were rebuked, when thus enraged beyond measure at the contumely of the Samaritans: in order that they might learn that as ministers of the divine tidings, they must rather be full of longsuffering and gentleness; not revengeful; not given to wrath, nor savagely attacking those who offend them.
And that the ministers of God’s message were longsuffering, Paul teaches us, saying, “For I think that God hath set forth us the apostles last, as it were, condemned to death; for we are made a spectacle unto the world, and to angels, and to men. Being reviled, we bless; being defamed, we persuade: we have become the offscouring of the world; the refuse of all men up to this day.” He wrote also to others, or rather to all who had not yet received Christ in them, but, so to speak, were still afflicted with the pride of the Samaritans: “We pray you in Christ’s stead, be ye reconciled to God.”
Great, therefore, is the benefit of the gospel lessons to those who are truly perfect in mind; and may we also, taking them unto ourselves, benefit our souls, ever praising Christ the Saviour of all: by Whom and with Whom to God the Father be praise and dominion, with the Holy Spirit, for ever and ever, Amen.