Bible Dictionary
UNQUENCHABLE FIRE
Unquenchable fire is a biblical image of the abiding and fearful judgment of the wicked. Jesus and John the Baptist use it in connection with Gehenna and the final separation of the righteous from the wicked.
Biblical Usage
John the Baptist uses the image for the burning of the chaff in Messianic judgment. Jesus applies it to Gehenna and speaks of fire that is not quenched; related New Testament passages associate fire with final punishment.