Jeremiah foresees a day of judgment coming upon Israel because the “sons of Judah… have set their detestable things in the house that is called by my name, to defile it”, and have sacrificed their children in Topheth, which is in the Valley of the Son of Hinnom (Jer. 7:30).
The Babylonian army will besiege the city, and the dead will be buried in Topheth or strewn across the Valley of the Son of Hinnom to be eaten by birds and beasts, because there is no burial space left in the city. The valley will be renamed the Valley of Slaughter (Jer. 7:30-34; 19:4-9). The city will be a horror, a thing to be hissed at by passers-by.
Jesus predicts the same appalling fate for Jerusalem when he threatens the Pharisees with the judgment of the Valley of the Son of Hinnom—the judgment of Gehenna (Matt. 23:33).
If Israel does not mend its ways, it will suffer invasion and war, and the dead again will be strewn in the valleys outside the walls for lack of burial space in the city, “where their worm does not die and the fire is not quenched” (Mk. 9:48; cf. Is. 66:24). Isaiah has imagined the unburied corpses of those who rebelled against YHWH as a permanent record of judgment.
It would be better for Jesus’ contemporaries to lose an eye or a hand than suffer the catastrophe that Jesus sees coming.
But when Jeremiah pictures a return from captivity and the establishment of a “new covenant with the house of Israel and the house of Judah”, when God will forgive their iniquities and write his Law on their hearts, he also foresees an end to Gehenna:
The whole valley of the dead bodies and the ashes, and all the fields as far as the brook Kidron, to the corner of the Horse Gate toward the east, shall be sacred to the LORD. It shall not be plucked up or overthrown anymore forever. (Jer. 31:40)
The “city shall be rebuilt for the Lord”, and this unholy place, so closely associated with both desecration and judgment, will be made sacred to the Lord. Never again will God’s people suffer such a dreadful destruction. There will be no more hell.
I don’t think we find this thought in the Gospels, but Paul may have the argument in mind, more or less, when he writes:
There is therefore now no condemnation for those who are in Christ Jesus. For the law of the Spirit of life has set you free in Christ Jesus from the law of sin and death. (Rom 8:1–2)
The Law condemns persistently sinful Israel to the sort of destruction that Jesus had envisaged (cf. Rom. 9:22), but those who are “in Christ Jesus” live according to the Spirit and there is no such condemnation for them.
Paul does not invoke the same concrete prophetic narrative—I think because here he has primarily diaspora Judaism in view rather than Jerusalem. Also Paul, like Jesus, is concerned less with the eventual restored state of the people of God than with the difficult historical journey that those in Christ must make in order to get to that place of restoration. In this argument, those who are heirs with Christ are those who suffer as he suffered, and who will be glorified as he was glorified—the martyrs (Rom. 8:17).
But with this new life in the Spirit there is an end to the “hell”—the judgment of Gehenna, the destruction of a people—that so preoccupied Jesus. It won’t happen again.