Here some good explanations from Thomas Allin's Christ Triumphant
"THAT YOUR WHOLE BODY SHOULD BE CAST INTO HELL." S. Matt. v. 29,30, and xviii. 8,9.
These passages are so similar that they may be considered together, and may be compared with S. Mark ix. 43-50, where a full comment is given. The "hell" of the text is "Gehenna, " and in ch. xviii. 8. 9, "hellfire" is the fire of Gehenna, and everlasting fire is aeonian fire.
"AND THESE SHALL GO AWAY INTO EVERLASTING PUNISHMENT, BUT THE RIGHTEOUS INTO LIFE ETERNAL." S. Matt. xxv. 46.
This text, if fairly translated, seems to require an interpretation quite distinct from that of the popular theology, and opposed to it. (a) "Everlasting" and "eternal" represent aionios, and mean "of or belonging to an age " - aeonian. (b) If a punishment absolutely endless were intended it seems unaccountable that a word should be used which habitually does not mean endless, but the opposite. (c) The word translated punishment means pruning, i.e., corrective punishment, and should be so rendered. (d) So that which is threatened seems the opposite of our popular hell; it is a corrective process, "proper to the age " - or "ages." (e) And of this beneficent purpose there is a hint, often unnoticed, in the term applied to those on the left hand, it is properly "kids" or "kidlings," a diminutive, implying a certain affection. And so for the paschal offering a kid was eligible (Ex. xi. 5) equally with a lamb; and in the Catacombs the Good Shepherd is at times depicted as bearing home on His shoulders A KID, not a lamb, i.e., a GOAT, not a sheep. (f) Nor must we forget that, in Rev. xx. 11, the throne of judgment is WHITE - the sign of peace and amity. But it is said that the same word is applied to the happiness of the saved and to the punishment of the lost; and that, if it does not mean endless in the latter case, the bliss of the redeemed is rendered uncertain. I reply (I.) even were it so, we are not at liberty to mistranslate, but (II.) in fact it is certainly not so. True, the text does assign an aeonian penalty and an aeonian reward, but this leaves perfectly open the whole question of the precise duration of either. For the term aeonian is quite indefinite, it does not touch the question of the limit of time; it simply teaches that both reward and penalty go on to a future age or ages. The question what will happen after this age or ages is not raised in this passage. (g) I have in these comments made two assumptions both very doubtful, and both favorable to the traditional creed.
* It must be noted that the endlessness of the happiness of the Redeemed depends, not on any meaning we assign to aionios. but on its own intrinsic nature, as resulting from union with Him, Who is endless life; and on texts easily to be found elsewhere, e.g., he that does the will of God abides for ever, 1 Jno. ii. 17; Because I live you shall live also, S. Jno. xiv. 19; If a man keep My saying he shall never taste of death. - S. Jno. viii. 51, cf v. 35. Compare Ps. cii. 28.
(I.) I have assumed the reference of aionios to time, which is not capable of proof; for with perfect fairness it may have here that spiritual, ethical meaning it unquestionably at times has in the New Testament; and the meaning then would be, that just and unjust pass into aeonian, i.e., spiritual states of punishment and bliss respectively. (II.) I have assumed the primary reference of this passage to the final Judgment, but that is most improbable; for these words close a continuous discourse extending over chapters xxiv-v. (which our division into chapters obscures.) There is no break throughout. And the question of the disciples, in ch. xxiv., is not about the end of the "world," but of the "age". Thus, if we divest ourselves of traditional impressions, and take Scripture itself as our guide, we see that it is not fair to refer to a distant future, that judgment of which Christ Himself says distinctly, (ch. xxiv. 34,) that ALL THE THINGS He is speaking of should be fulfilled before the passing away of the then generation; and which finds a perfectly natural fulfillment in the terrible calamities, consequent on the fall of Jerusalem, and the end of the (Jewish) age (as these events would be described in Eastern metaphor). And indeed our Lord's words, "all the nations" v. 32, seem to refer to national judgments, and to indicate, in dramatic form, the principle on which judgment falls on nations; certainly increasing reflection makes this reference seem increasingly probable.
"WHOSOEVER SHALL SAY, YOU FOOL, SHALL BE IN DANGER OF HELLFIRE." S. Matt. v. 22.
The popular interpretation reduces these words to an absurdity. "It is incredible that to call a man a fool should be so much a worse crime than to call him Raca, that, whereas for the one offense men are to be brought before a court of justice, for the other they are to be damned to an everlasting torment." - Salv. Mund. The hellfire of this passage is the fire of "Gehenna."
IMPORTANT FACTS ABOUT GEHENNA from the The Bible Hell
1. Gehenna was a well-known locality near Jerusalem, and ought no more to be translated Hell, than should Sodom or Gomorrah. See Josh. 15: 8; II Kings 17: 10; II Chron. 28: 3; Jer. 7: 31, 32; 19: 2.
2. Gehenna is never employed in the Old Testament to mean anything else than the place with which every Jew was familiar.
3. The word should have been left untranslated as it is in some versions, and it would not be misunderstood. It was not misunderstood by the Jews to whom Jesus addressed it. Walter Balfour well says: "What meaning would the Jews who were familiar with this word, and knew it to signify the valley of Hinnom, be likely to attach to it when they heard it used by our Lord? Would they, contrary to all former usage, transfer its meaning from a place with whose locality and history they had been familiar from their infancy, to a place of misery in another world? This conclusion is certainly inadmissible. By what rule of interpretation, then, can we arrive at the conclusion that this word means a place of misery and death?"
4. The French Bible, the Emphatic Diaglott, Improved Version, Wakefield's Translation and Newcomb's retain the proper noun, Gehenna, the name of a place as well-known as Babylon.
5. Gehenna is never mentioned in the Apocrypha as a place of future punishment as it would have been had such been its meaning before and at the time of Christ.
6. No Jewish writer, such as Josephus or Philo, ever uses it as the name of a place of future punishment, as they would have done had such then been its meaning.
7. No classic Greek author ever alludes to it and therefore it was a Jewish locality, purely.
8. The first Jewish writer who ever names it as a place of future punishment is Jonathan Ben Uzziel who wrote, according to various authorities, from the second to the eighth century, A. D.
9. The first Christian writer who calls Hell Gehenna is Justin Martyr who wrote about A. D. 150.
10. Neither Christ nor his apostles ever named it to Gentiles, but only to Jews which proves it a locality only known to Jews, whereas, if it were a place of punishment after death for sinners, it would have been preached to Gentiles as well as Jews.
11. It was only referred to twelve times on eight occasions in all the ministry of Christ and the apostles, and in the Gospels and Epistles. Were they faithful to their mission to say no more than this on so vital a theme as an endless Hell, if they intended to teach it?
12. Only Jesus and James ever named it. Neither Paul, John, Peter nor Jude ever employ it. Would they not have warned sinners concerning it, if there were a Gehenna of torment after death?
13. Paul says he "shunned not to declare the whole counsel of God," and yet though he was the great preacher of the Gospel to the Gentiles he never told them that Gehenna is a place of after-death punishment. Would he not have repeatedly warned sinners against it were there such a place?
Dr. Thayer significantly remarks: "The Savior and James are the only persons in all the New Testament who use the word. John Baptist, who preached to the most wicked of men did not use it once. Paul wrote fourteen epistles and yet never once mentions it. Peter does not name it, nor Jude; and John, who wrote the gospel, three epistles, and the Book of Revelations, never employs it in a single instance. Now if Gehenna or Hell really reveals the terrible fact of endless woe, how can we account for this strange silence? How is it possible, if they knew its meaning and believed it a part of Christ's teaching that they should not have used it a hundred or a thousand times, instead of never using it at all; especially when we consider the infinite interests involved? The Book of Acts contains the record of the apostolic preaching,and the history of the first planting of the church among the Jews and Gentiles, and embraces a period of thirty years from the ascension of Christ. In all this history, in all this preaching of the disciples and apostles of Jesus there is no mention of Gehenna. In thirty years of missionary effort these men of God, addressing people of all characters and nations never under any circumstances threaten them with the torments of Gehenna or allude to it in the most distant manner! In the face of such a fact as this can any man believe that Gehenna signifies endless punishment and that this is part of divine revelation, a part of the Gospel message to the world? These considerations show how impossible it is to establish the doctrine in review on the word Gehenna. All the facts are against the supposition that the term was used by Christ or his disciples in the sense of endless punishment. There is not the least hint of any such meaning attached to it, nor the slightest preparatory notice that any such new revelation was to be looked for in this old familiar word."
14. Jesus never uttered it to unbelieving Jews, nor to anybody but his disciples, but twice (Matt. 23: 15-33) during his entire ministry, nor but four times in all. If it were the final abode of unhappy millions, would not his warnings abound with exhortations to avoid it?
15. Jesus never warned unbelievers against it but once in all his ministry (Matt. 23: 33) and he immediately explained it as about to come in this life.
16. If Gehenna is the name of Hell then men's bodies are burned there as well as their souls. Matt. 5: 29; 18: 9.
17. If it be the name of endless torment, then literal fire is the sinner's punishment. Mark 9: 43-48.
18. Salvation is never said to be from Gehenna.
19. Gehenna is never said to be of endless duration nor spoken of as destined to last forever, so that even admitting the popular ideas of its existence after death it gives no support to the idea of endless torment.
20. Clement, a Universalist, used Gehenna to describe his ideas of punishment. He was one of the earliest of the Christian Fathers. The word did not then denote endless punishment.
21. A shameful death or severe punishment in this life was at the time of Christ denominated Gehenna (Schleusner, Canon Farrar and others), and there is no evidence that Gehenna meant anything else at the time of Christ.
I also recommend this short article
as a good basis for further studies.