Maybe this will help some here to understand this idea...
Quotes are taken from J.W. Hanson's, "Bible Threatenings Explained."
Hanson wrote, "Gehenna was a well-known valley, near Jerusalem, in which the Jews in their idolatrous days had sacrificed their children to the idol Moloch, in consequence of which it was condemned to receive the offal and refuse and sewage of the city, and into which the bodies of malefactors were cast and where to destroy the odor and pestilential influences, continual fires were kept burning. Here fire, smoke, worms bred by the corruption, and other repulsive features, rendered the place a horrible one, in the eyes of the Jews. It was locality with which they were as well acquainted as they were with any place in or around the city. But in process of time Gehenna came to be an emblem of the consequences of sin, and to be employed figuratively by the Jews, to denote those consequences. But always in this world. The Jews never used it to mean torment after death, until long after Christ. That the word had not the meaning of post-mortem torment when our Savior used it."
Hanson wrote, "Every Bible reference is to this world. Only Jesus and James ever named it. Neither Paul, John, Peter nor Jude ever employed it. Would they not have warned sinners concerning it, if there were a Gehenna of torment after death?" 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 preached to Gentiles as well as to Jews.
"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 a part of divine revelation, a part of the gospel message to the world?"
"Now if endless punishment awaits millions of the human race, and if it is denoted by this word, is it possible that only David, Isaiah, Jeremiah, Daniel, and Malachi use the word to define punishment, in all less than a dozen times, while Job, Moses, Joshua, Ruth, Ezra, Nehemiah, Esther, Solomon, Ezekiel, Hosea, Joel, Amos, Obadiah, Jonah, Micah, Hahum, Habbakkuk, Zephaniah, Haggai and Zachariah never employed it thus? Such silence is criminal, on the popular hypothesis. These holy men should and would have made every sentence bristle with the word, and thus have borne the awful message to the soul with an emphasis that could neither be resisted or disputed. The fact that the word is so seldom, and by so few applied to punishment, and never in the Old Testament to punishment beyond death, demonstrates that it cannot mean endless."
Unquenchable:
Lev. 6:12-13; 2 King 22:17; 2 Chron. 34:25; Isa. 34:10; 42:3; 43:3; 66:24; Jer 4:4; 7:20; 17:27; 21:12; Exek. 20:47; Matt. 12:20; Mark 9:43-48
Hanson wrote, "Many suppose that the words "unquenchable fire" mean a fire of endless duration, whereas, it is a fire that cannot be quenched until its purpose is accomplished. The meaning is, not that the fire was endless, but that it was not quenched,--it continued to burn--until all the material was destroyed. So the judgments of God on the Jews were effectually done -- the nation was completely devastated and destroyed. They were like chaff of the summer threshing floor in the consuming fire of God's judgment."
"We see the word everlasting applied to God's covenant with the Jews; to the priesthood of Aaron; to the statutes of Moses; to the time the Jews were to possess the land of Canaan; to the mountains and hills; and to the doors of the Jewish temple. We see the word forever applied to the duration of a man's earthly existence; to the time a child was to abide in the temple; to the continuance of Gehazi's leprosy; to the duration of the life of David; to the duration of a king's life; to the duration of the earth; to the time the Jews were to possess the land of Canaan; to the time they were to dwell in Jerusalem; to the time a servant was to abide with his master; to the time Jerusalem was to remain a city; to the duration of the Jewish temple; to the laws and ordinances of Moses; to the time David was to be king over Israel; to the throne of Solomon; to the stones that were set up at Jordan; to the time the righteous were to inhabit the earth; and to the time Jonah was in the fish's belly. We find the phrase forever and ever applied to the hosts of heaven, or the sun, moon, and stars; to a writing contained in a book; to the smoke that went up from the burning land of Idumea; and to the time the Jews were to dwell in Judea. We find the word never applied to the time the fire was to burn on the Jewish altar; to the time the sword was to remain in the house of David; to God's covenant with the Jews; to the time the Jews should not experience shame; to the time the house of David was to reign over Israel; to the time the Jews were not to open their mouths because of their shame; to the time those who fell by death should remain in their fallen state; and to the time judgment was not executed.
But the law covenant is abolished; the priesthood of Aaron and his sons has ceased; the ordinances, and laws, and statutes of Moses are abrogated; the Jews have long since been dispossessed of the land of Canaan, have been driven from Judea, and God has brought upon them a reproach and a shame; the man to the duration of whose life the word forever was applied is dead; David is dead, and has ceased to reign over Israel; the throne of Solomon no longer exists; the Jewish temple is demolished, and Jerusalem has been overthrown, so that there is not left "one stone upon another;" the servants of the Jews have been freed from their masters; Gehazi is dead, and no one believes he carried his leprosy with him into the future world; the stones that were set up at Jordan have been removed, and the smoke that went up from the burning land of Idumea has ceased to ascend; the righteous do not inherit the earth endlessly, and no one believes that the mountains and hills, as such, are indestructible; the fire that burnt on the Jewish altar has long since ceased to burn; judgment has been executed; and no Christian believes that those who fall by death will never be awakened from their slumbers. Now, as these words are used in this limited sense in the Scriptures, why should it be supposed that they express endless duration when applied to punishment?" (Thomas B. Thayer - The Origin and History of the Doctrine of Endless Punishment). See Everlasting. - Gen. 17:7, 8, 13; 48:4; 49:26; Exod. 40:15; LeV 16:34; Numb. 25:13; Ps. 24:7; Hab. 3:6. Forever. - Deut. 15:17; 1 Sam. 1:22; 27:12; LeV 25:46; 2 Kings 5:27; Job 41:4; 1 Kings 1:31; Neh. 2:3; Dan. 2:4; Exod. 14:13; Eccl 1:4; Ps. 104:5; 78:69; Ezek. 37:25; Gen. 13:15; Exod. 32:13; Josh. 14:9; 1 Chron. 23:25; Jer. 17:25; Ps. 48:8; Jer. 31:40; 1 Kings 8:13; Numb. 10:8;18:23; 1 Chron. 28:4; 1 Kings 9:5; Josh. 4:7; Jonah 2:6; Ps. 37:29. Forever and ever. - Ps. 148:5, 6; Isa. 30:8; 34:10; Jer. 7:7; 25:5. Never. - LeV 6:13; 2 Sam. 12:10; Judges 2:1; Joel 2:26, 27; Jer. 33:17; Ezek. 16:63; Amos 8:14; Hab. 1:4.
Hanson wrote, "All these and numerous other eternal, everlasting things -- things that were to last forever, and to which the various aionion words are applied--have now ended, and if these hundreds of instances must denote limited duration why should the few times in which punishments are spoken of have any other meaning? Even if endless duration were the intrinsic meaning of the word, all intelligent readers of the Bible would perceive that the word must be employed to denote limited duration in the passages above cited. And surely in the very few times in which it is connected with punishment it must have a similar meaning."