Among the Israelites a burial in a memorial tomb symbolized righteousness for the one being buried. This was not scriptural, just their way of showing that they considered a person to be worthy of a resurrection. So for a corpse to be condemned into the seemingly eternal fire of a trash dump was the opposite of that. For someone to be thrown into Gehenna symbolized the resident’s abhorrence of this person. If the fire didn’t burn up their corpses, they were on display for everyone to see the maggots eating them. This was the place where the worms never died and the fire never went out. You could always find both of these things in this valley. (Isaiah 66:24) This is something that these guys were familiar with, so what better for Jesus to reference in order to get his point across? Jesus used this trash dump as an illustration to explain how everything that is no part of his Father’s kingdom will be shamefully discarded in the same way.
When Jesus spoke of people who were fit for Gehenna the fire, he was speaking of shameful people who were not deemed worthy of a resurrection. There were only twelve instances in the Greek Scriptures speaking of Gehenna. In the manuscripts that render Sheol, Hades, and Gehenna as “hell,” these are the twelve scriptures where hell gets completely different characteristics applied to it, misleading many people.
To be in Sheol/Hades meant to be in a good place in God’s memory and promised a resurrection, as seen at Revelation 20:13. On the other hand, to be in Gehenna meant to be in a bad place in God’s memory and therefore denied a resurrection, as seen at Matthew 23:15. Yet both of these places with absolute opposite meanings are commonly rendered as “hell.”
Brimstone, a.k.a. sulphur, was frequently added to Gehenna to heat the fire up and to keep it going, especially when there was nothing to burn. Sulphur gets really hot and burns for a long time, and it takes temperatures of 3000-5000 degrees to burn a human body quickly and completely, so you can appreciate why it was frequently added to completely destroy corpses. Gehenna was later described as “the Lake of Fire and Sulphur.” Revelation 21:8
God even spoke of living people being burned in Gehenna as sacrifice and said that he never even considered doing such a thing –
They have built the high places of Topheth in the Valley of Hinnom in order to burn their sons and daughters in the fire, a thing I did not command; I never entertained the thought. Jeremiah 7:31
Also - They have built high places to Baal on which to burn their children in the fire as burnt offerings to Baal, something I have never commanded or mentioned; I never entertained the thought. Jeremiah 19:5