'The Gehenna (Gey Hinnom): Background —

Gehenna: the valley of the son of Hinnom

The Hebrew Gei-Hinnom in the Jewish Bible was an abhorrent place to God (Jesus) where the Jews engaged in the practice of burning their children. Jeremiah 7:31 unequivocally attests to that. Jeremiah had witnessed the destruction of Gei-Hinnom by King Josiah. Jeremiah started his prophetic career in the 13th year of the reign of King Josiah (Jeremiah 1:2). Five years later, King Josiah, in his 18th year, decided to do some restoration work at the Temple. The High Priest found the lost book of the Law of Moses and it got to King Josiah to whom it was read. He vowed to follow the book of the Law word for word. This vow brought him to accomplish the prophecy which foretold the destruction of idolatry in the country and to Gei-Hinnom (Gehenna) where, according to the Jewish Bible) the Jews burn their children to an idol (2 Kings 22 and 23, Jeremiah 7:31). God (Jesus) sent word to Josiah through the married Prophetess Huldah that He would not bring disaster to Israel while he was alive.

Josiah was named by his name to destroy the idolatry way before he was born as the prophecy was made to King Jeroboam of Israel (1 Kings 13), the first king of the division of the kingdom of Solomon into two kingdoms. Josiah came at the last leg of the kingdom of Judah, some 300 years after (2 kings 23:16-18), to destroy the foundations of idolatry and Gehenna (valley of Hinnom)!

In our modern world, many "Christians" still practice idolatry by christening the likes of Moloch, even though the Author of Christianity forbids it in Moses and the Prophets. 

 

And have built the high places of Tophet, That [are] in the valley of the son of Hinnom, To burn their sons and their daughters with fire, Which I did not command, Nor did it come up on My heart." Jeremiah 7:31 CLV

and himself [Ahaz] has made perfume in the valley of the son of Hinnom, and burns his sons with fire according to the abominations of the nations that Yahweh dispossessed from the presence of the sons of Israel, 2 Chronicles 28:3 CLV

And he [Manasseh] has caused his sons to pass over through fire in the valley of the son of Hinnom, and observed clouds and used enchantments and witchcraft, and dealt with a familiar spirit, and a wizard; he has multiplied to do the evil thing in the eyes of Yahweh, to provoke him to anger. 2 Chronicles 33:6 CLV

And they build the high places of Baal, that [are] in the valley of the son of Hinnom, to cause their sons and their daughters to pass through to Molech, which I did not command them, nor did it come up on my heart to do this abomination, so as to cause Judah to sin." Jeremiah 32:35 CLV

When Jesus speaks of Gehenna as a place of 'unquenchable fire', the fiery slaughter imagery resonates well with His Jewish audience. 

Jeremiah 7:30-33 ASV

(30)  For the children of Judah have done that which is evil in my sight, saith Jehovah: they have set their abominations in the house which is called by my name, to defile it.
(31)  And they have built the high places of Topheth, which is in the valley of the son of Hinnom, to burn their sons and their daughters in the fire; which I commanded not, neither came it into my mind.
(32)  Therefore, behold, the days come, saith Jehovah, that it shall no more be called Topheth, nor The valley of the son of Hinnom, but The valley of Slaughter: for they shall bury in Topheth, till there be no place to bury.
(33)  And the dead bodies of this people shall be food for the birds of the heavens, and for the beasts of the earth; and none shall frighten them away.