For example, in Ezk. 20.47-48, God promised national judgment on Israel:
Hear the word of the Lord: Thus says the Lord God, Behold, I am about to kindle a fire in you, and it shall consume every green tree in you, as well as every dry tree; the blazing flame will not be quenched, and the whole surface from south to north will be burned by it. And all flesh will see that I, the Lord, have kindled it; it shall not be quenched.
In Amos 5.5-6, we have the same language used of national judgment on Israel again. God had promised a similar judgment on the northern kingdom at the hands of the Assyrians, fulfilled in 722 B.C.:
Seek the Lord that you may live, lest He break forth like a fire, O house of Joseph, and it consume with none to quench it for Bethel.
In Isa. 66.15-16, 24, Isaiah spoke of New Jerusalem's enemies being burned with unquenchable fire, as he spoke of the destruction of Jerusalem by the Romans in A.D. 70:
For behold, the Lord will come in fire, and His chariots like the whirlwind, to render His anger with fury, and His rebuke with flames of fire. For the Lord will execute judgment by fire, and by His sword on all flesh. And those slain by the Lord will be many....Then they shall go forth and look on the corpses of the men who have transgressed against Me. For their worm shall not die, and their fire shall not be quenched; and they shall be an abhorrence to all mankind.
In Jer. 21.10-12, we read of Babylon's burning Jerusalem with unquenchable fire, a national judgment fulfilled in 586 B.C.:
For I have set My face against this city for harm and not for good, declares the Lord. It will be given into the hand of the king of Babylon, and he will burn it with fire. Then say to the household of the king of Judah, Hear the word of the Lord, O house of David, thus says the Lord: Administer justice every morning; and deliver the person who has been robbed from the power of the oppressor. That My wrath may not go forth like fire and burn with none to extinguish it, because of the evil of their deeds.
Again, at the time John the Baptist and Jesus used this language in the gospels, the Bible had only used it of national judgments.
Fire That Is Not Quenched
The same thing is true of this expression. In Jer. 4.4, Jeremiah used it of the destruction of Jerusalem. In Jer. 21.12, he used it to describe the destruction of the house of David. In Amos 5.5, 6, Amos used it of the destruction of Jerusalem. In II K. 22.17, it's used of the destruction of Judah. In Isa. 34.10, Isaiah used it of the destruction of Edom, and in Isa. 66.24, he used it of the destruction of the enemies of the Messiah's people. See also Jer. 7.20, 17.27, where Jeremiah used it of the destruction of Judah, and Ezk. 20.47-48, where Ezekiel spoke of God's destruction of Jerusalem.