If “salted” carries the idea of being benefitted in some way by trying circumstances (“fire”) then yes, I do think all will ultimately be benefitted by the trying circumstances of this life - and I have no problem understanding Mark 9:49 as a general (even universal) principle that Christ is affirming. But I see absolutely no evidence at all to understand “Gehenna” as referring to a post-mortem (or post-resurrection) purgatorial experience that is inclusive of, and applicable to, all people of every generation. If the expression “all will be salted with fire” is simply a general principle (which appears to me to be the case), then the judgment of “Gehenna fire” is likely just one example of a trying situation by which people (in this case, the Jews of that generation, both believers and unbelievers) were “salted.” It is this judgment (and all the trying circumstances that led up to it) that most concerned Jesus’ disciples during that generation. In the immediate context, this judgment is spoken of as being concurrent with the time when people would be entering the kingdom of God - i.e., when the age of the Messianic reign began. This blessing of inheriting the kingdom of God is also called “life,” which of course is short for what is elsewhere called eternal life, or the “life of the age.” And to what “age” does the “life” pertain or belong? Answer: the age that was to begin whenever the age in which Christ and his disciples were living, ended (Matt 24:3). It was this then-future age that was referred to as “the age to come,” and was associated with “the life of the age” (Mark 10:30; Luke 18:30; Heb 6:6; etc.). And what age was “to come” at this time? Answer: the age of the Messianic reign, which was to commence with Christ’s coming in his kingdom before that generation passed away (Matt 24-25) - i.e., when the kingdom “came with power” within the lifetime of “some” (tis, one or more) of those who heard Christ prophesy of it (Mark 9:1; Matt 16:28)
Now, the first time Christ speaks of Gehenna is in Matt 5:22. It is significant that neither Christ nor the Gospel writer give any explanation of the word, which (being an OT word with meaning already attached to it) strongly suggests that those familiar with the Hebrew Scriptures would be able to figure out what Christ is talking about without doing much (or any) speculating or extra-biblical research. The verse reads, “But I say to you that everyone who is angry with his brother will be liable to judgment; whoever insults his brother will be liable to the council; and whoever says, ‘You fool!’ will be liable to the Gehenna of fire.”
So let’s see…those who are angry with his brother would be liable to judgment [probably a reference to the council of twenty three magistrates], while whoever insults his brother would be liable to the council *. So far, so good; these are both examples of some form of temporal judgment with which Jesus’ disciples would have been familiar. But according to the popular universalist understanding of “Gehenna” (which I admit is not as crazy as the traditional, orthodox view!), those who say “Raca!” are liable not to an even more severe temporal punishment (which would make sense in the context), but to some indefinitely long, post-mortem purgatorial process that all people may experience in some way or another after death and/or the resurrection. But there is simply no Scriptural evidence that this is what Jesus was talking about when he referred to Gehenna. I mean, even setting aside Jer 19 as Scriptural evidence for how Christ employed the word, it seems pretty evident from passages like Matthew 23:32-36 that Christ was referring to a national judgment that was soon to fall upon the generation in which he lived:
"Fill up, then, the measure of your fathers. You serpents, you brood of vipers, how are you to escape being sentenced to Gehenna? Therefore I send you prophets and wise men and scribes, some of whom you will kill and crucify, and some you will flog in your synagogues and persecute from town to town, so that on you may come all the righteous blood shed on earth, from the blood of innocent Abel to the blood of Zechariah the son of Barachiah, whom you murdered between the sanctuary and the altar. Truly, I say to you, all these things will come upon this generation.
And what does Jesus immediately go on to speak about to the end of this chapter (and on into the next)? Answer: the national judgment that God was about to bring upon Israel.
But there’s no Biblical precedent for ascribing to “Gehenna” anything other than either its literal meaning (Josh 15:8, 18:16; 2 Kings 23:10; 2 Chron 28:3, 33:6; Neh 11:30; Jer 7:31, 32, 19:2, 6, 32:35) or the figurative, emblematic meaning that is attached to it in Jer 19 by divine authority. The objection that there is no reference to “fire” in Jer 19 is forceless, since there is very strong precedent for understanding “fire” as a metaphor for God’s wrath manifested in temporal judgments upon people or nations (Deut 29:23-24; 32:22; 2 Sam 22:9, 13; Job 18:15; Psalm 11:6; 21:9; 29:7; 50:3; 68:2; 78:21; 79:5; 83:13-15; 89:46; 97:3; Isaiah 9:19; 10:17; 30:27-33; 34:9-10; 42:24-25; 47:14; 66:15-16, 24; Jer 4:4; 17:4, 27; 21:10-12; 48:45; Lam 2:3-4; 4:11; Ezekiel 5:1-4; 21:31; 22:17-22, 31; 38:22; Amos 1:4, 7, 10, 12, 14; 2:2, 5; 5:6; Obadiah 1:18; Nahum 1:6; Zeph 3:8; Zech 13:9; Mal 3:2) - which ties in perfectly with what I think Jesus meant when he made reference to “Gehenna.” But since many don’t think that Jesus is referring to a temporal judgment upon unfaithful Israel by his use of “Gehenna” (for which, again, there is precedence in the OT) I would like to know what source informs one’s understanding that Jesus is referring to something else (e.g., some purgatorial process in a future state of existence)? Where does one get this information from? The Jewish Targums? All I know is that it’s definitely not from any inspired source.
And yes, Jesus is quoting the final verses of Isaiah, but that’s even more evidence that a temporal judgment is in view (as opposed to some post-mortem, indefinitely long purification process that somehow cleanses people from sin and reconciles all remaining rebels to God). As you’re well aware, those being consumed by fire and devoured by maggots are not “disembodied spirits” or resurrected immortals, for Isaiah writes: “And they shall go out and look on the dead bodies of the men who have rebelled against me. For their worm shall not die, their fire shall not be quenched, and they shall be an abhorrence to all flesh.”
Moreover, the “unquenchable fire” of which we read in Isaiah 66 and Mark 9 is simply typical OT language used to describe temporal judgments upon nations (especially Israel!!):
Its streams shall be turned into pitch, and its dust into brimstone; its land shall become burning pitch. It shall not be quenched night or day; its smoke shall ascend forever. From generation to generation it shall lie waste; no one shall pass through it forever and ever. Isaiah 34:9-10
"But if you will not heed Me to hallow the Sabbath day, such as not carrying a burden when entering the gates of Jerusalem on the Sabbath day, then I will kindle a fire in its gates, and it shall devour the palaces of Jerusalem, and it shall not be quenched." Jeremiah 17:27
“For I have set me face against this city * for harm and not for good, declares the LORD: it shall be given into the hand of the king of Babylon, and he shall burn it with fire.* And to the house of the king of Judah say, ‘Hear the word of the LORD, O house of David! Thus says the LORD: ‘Execute justice in the morning, and deliver from the hand of the oppressor him who has been robbed, lest my wrath go forth like fire, and burn with none to quench it, because of your evil deeds.’” Jeremiah 21:10-12 (cf. v. 14)
"Behold, I will kindle a fire in you, and it shall devour every green tree and every dry tree in you; the blazing flame shall not be quenched, and all faces from the south to the north shall be scorched by it. All flesh shall see that I, the LORD, have kindled it; it shall not be quenched." Ezekiel 20:47-48
“Seek the LORD and live, lest he break out like fire in the house of Joseph, and it devour, with none to quench it for Bethel, O you who turn justice to wormwood and cast down righteousness to the earth!” Amos 5:6-7
And in Isaiah 33:14 we read: "The sinners in Zion are afraid; trembling has seized the godless: ‘Who among us can dwell with the consuming fire? Who among us can dwell with everlasting (olam) burnings?’" Here, it is the sinners and godless in Zion who are said to be exposed to the fearsome judgment of “consuming fire” and “everlasting burnings.” And earlier, the prophet had said that God’s “fire is in Zion,” and his “furnace is in Jerusalem” (31:9).
But who constitutes the “Jerusalem” whose enemies God is represented as judging in the last chapter of Isaiah? It is surely not inclusive of Israelites like those against whom Jeremiah prophesied in Jer 19, who brought down judgment upon themselves because of their pagan abominations against God (Jer 19:4-5). No, the “Jerusalem” in view here is constituted solely by the believing Jewish remnant who, along with all believing Gentiles, were included as citizens of the “New Jerusalem” of Rev 21-22 when the age of the Messianic reign began in 70 - i.e., it constitutes those whom Paul calls the “Israel of God” (Gal 6:16). And Jerusalem’s “pagan enemies” against whom God “fought” and avenged his people - who were they? Shockingly, they turned out to be of their own brethren - i.e., the unbelieving Jewish majority of whom Paul speaks in Rom 9-11. This “Jerusalem” - i.e., the one that God didn’t “fight for” but actually fought against - is described in Zech 14:
“Behold, a day is coming for the LORD, when the spoil taken from you will be divided in your midst. For I will gather all the nations against Jerusalem to battle and the city shall be taken and the houses plundered and the women raped. Half of the city shall go out into exile, but the rest of the people shall not be cut off from the city.” Zech 14:1-2 (cf. Luke 21:20-24).
Again, I don’t think that is the case at all. Neither Christ’s words in Mark 9:49 nor his reference to “Gehenna” and “unquenchable fire” need be seen as evidence that his focus is “much wider” than the overthrow of the nation of Israel in 70. Nowhere in the OT does “Gehenna” have anything at all to do with any nation other than Israel. And the so-called “rebel Gentiles” (as you say) of Isa 66:24 actually turned out to be rebel Jews whose abominations against God surpassed even those of the most depraved Gentiles that the world had ever seen. Which is one of the great ironic (and tragic) twists of the story of redemptive history: those who prided themselves on being “the people of God” were ultimately severed from their own “olive tree” and “cast out” as his people (Rom 9-11), while those who had formerly been considered “not-my-people” (the heathen) became the new people of God (Hos 2:23) by embracing Jesus as the Messiah. And while this dramatic “turn of the tables” was certainly prophesied in the OT, Isaiah (especially toward the end of his prophetic work) is largely concerned with the believing remnant within national Israel who were to inherit the Messianic kingdom (i.e., when the kingdom “came with power” within the lifetimes of some of Christ’s contemporaries - Mark 9:1; Matt 16:28). It is this Jewish remnant which I believe constituted the “Jerusalem” of Isaiah 65:18ff (and elsewhere, like Isa 24:2), and which God defended and vindicated by casting apostate (dare I say pagan?) Jerusalem into “Gehenna fire.” Paul certainly realized that there had been a line drawn (so to speak) by God between unbelieving Israel and believing Israel; one group was identified with the then-present “Jerusalem” while the other was identified with the “Jerusalem above” (Gal 4:21-31), which (figuratively speaking) descended to earth when the former Jerusalem was destroyed. Those who had come to constitute the “Jerusalem above” were thus vindicated by God, while all who clung to the former Jerusalem ended up suffering severe judgment: “And Isaiah cries out concerning Israel: 'Though the number of the sons of Israel be as the sand of the sea, only a remnant of them will be saved, for the Lord will carry out his sentence upon the earth fully and without delay” (Rom 9:27-28).
I’m not seeing any “widescale cosmic level of punishment” in Mark 9 or in the two verses you referenced, and I don’t think appealing to the two verses from Isaiah you referenced helps buttress your position. Isaiah 1:31 is referring to a judgment upon Israel; the “both” refers to “the strong” and “his work” of the “faithful city” which had become a “whore” (v. 21) - i.e., Jerusalem. But even if God were talking about both rebel Jews and Gentiles being judged in this passage (or elsewhere), it would not make it a “widescale cosmic level of punishment,” or even a simultaneously occurring temporal judgment - it would simply mean that, just as one nation is judged, so shall another, irrespective of what nation it is. Moreover, just because similar language is applied to the downfall of Babylon doesn’t mean it must (or even can) refer to Gentiles being judged in Mark 9. Again, Gehenna, being a specifically Jewish word for a specifically Jewish location, most likely refers to a specifically Jewish judgment when employed by Christ in a figurative sense (as it does in Jer 19).
You ask, “and who would dare to say that the application of the same saying during the Sermon on the Mount, 5:29-30, was supposed to “specifically” refer to the forthcoming fall of Jerusalem?!” Answer: I would! Yes, Jesus is proclaiming a system of “kingdom ethics” applicable to both Jew and Gentile in all successive ages of redemptive history, but who, specifically, is Jesus addressing here? First century Jews! It is completely natural and appropriate that Jesus would add emphasis to his teaching during this discourse by referring to a judgment that concerned the very men to whom he spoke! If Jesus had wanted to speak of a future judgment upon the nation of Israel that was then approaching (a horrific judgment which would affect the lives of most of the people to whom he spoke during his “Sermon on the Mount”), what better word could he have possibly used?
On the other hand, I can’t even come up with a single Scriptural phrase - let alone a single word - that Christ could have used to refer to what many think “Gehenna” means. Since the OT is completely silent on the idea of post-mortem punishment (be it remedial or otherwise) - let alone a universal judgment/punishment in another state of existence - it would have required a completely new revelation from Christ (and most certainly would have required more than a single, unexplained word to express and sanction the radical idea that it was meant to convey).*