We didn’t find much interest in teaching the doctrine 0f Gehenna, in the Gospels. Let’s try the epistles.
Jude does a big line in warning about coming judgement.
Now I desire to remind you, though you are fully informed, that the Lord, who once for all saved a people out of the land of Egypt, afterward destroyed those who did not believe. And the angels who did not keep their own position, but left their proper dwelling, he has kept in eternal chains in deepest darkness for the judgment of the great Day. Likewise, Sodom and Gomorrah and the surrounding cities, which, in the same manner as they, indulged in sexual immorality and pursued unnatural lust, serve as an example, suffering a judgement of eternal fire. Jude 5-7.
These are three parallel examples of people experiencing judgement for sin. Jude connects them explicitly, with his ‘likewise’, inviting us to view them as parallel. Let’s do that.
The first refers to the destruction of the entire generation of Israelites who left Egypt at the exodus. From Numbers and Deuteronomy.
The second tells a story found in the apocryphal book 1 Enoch, which was popular in the times of the early church. Probably based on Genesis 6:1, it details the punishment in chains of angels who tried to cross-breed with humans. These are imprisoned in darkness until the day of judgement.
Their chains are described as eternal chains (Greek aidios). There is a lot of argument about what these ‘eternal’ words mean: aidios and aionios. Here it’s pretty clear that it doesn’t mean ‘everlasting’, since Jude specifically says that this state is temporary: ‘until the day of judgement.’
The third judgement is from Genesis 19. The destruction of those cities is describes as ‘a judgement of eternal fire.’ This is very similar to the prophetic usage of ‘eternal’ or ‘unquenchable’ fire, as an unstoppable fire that completely destroys. Clearly that is the sense of ‘eternal’ (Greek aionios) here, where the fire completely destroys the cities of the plain. This fire is not being viewed as ‘everlasting’. It is also a prophetic usage to treat Sodom and Gomorrah as the archetypal or exemplary recipients of God’s judgement against the wicked (cf. Isaiah 1:9, 13:19; Jeremiah 49:18; Amos 4:11).
This is a significant reference, showing yet another NT figure who can speak of ‘eternal fire’ with an OT sense, apparently not thinking of the intertestamental Gehenna tradition.
N.B. The NIV translation ‘they serve as an example of those who suffer the punishment of eternal fire’, is an interpretive translation that stretches the grammar a bit. There is no ‘of those who‘ in the Greek. You would only translate like this to avoid the annihilationist implications of Jude’s ‘eternal fire’ usage, i.e. the NIV translation arises out of assumptions that the NT teaches the doctrine: “Eternal fire = Hell”. The translation above is followed by many recent versions (NRSV, HCSB, NET, etc), and is a more natural rendering of the Greek original.