And now also the axe is laid unto the root of the trees: therefore every tree which bringeth not forth good fruit is hewn down, and cast into the fire. I indeed baptise you with water unto repentance: but he that cometh after me is mightier than I, whose shoes I am not worthy to bear: he shall baptise you with the Holy Ghost, and with fire: Whose fan is in his hand, and he will throughly purge his floor, and gather his wheat into the garner; but he will burn up the chaff with unquenchable fire.
In Jewish literature "fire" was a distinguishing feature of the final judgement. Fire and water are two great natural purifying agencies, and it is appropriate that both should be used to represent the regeneration of the heart. In a similar way, they are the two agencies by means of which God has purified, or will purify, this earth from sin and sinners (2 Peter 3:5-7). If men persistently cling to sin, they must eventually be consumed with it; how much better it is to permit the Holy Spirit to carry forward the purifying work now, while probation still lingers! A man will be either purged of sin or purged along with it. Paul said, "Fire shall try every man's work" (1 Cor. 3:13).
The sense in which Christ was to baptise with fire is not certain. This statement may have pointed forward to Pentecost, when the disciples were baptised with the Holy Ghost under the symbol of fire (Acts 2:3, 4). Again, it may refer to the fires of the last day, as may be implied by the parallel nature of Matt. 3:12 (see on v. 12). It may refer to the grace of God purifying the soul. Or, again, it may refer to the fiery trials that Peter speaks of as testing the Christian (1 Peter 4:12; cf. Luke 12:49, 50). Perhaps the words of John the Baptist include more than one aspect of the Bible symbolism relating to fire.
When John spoke of "unquenchable fire" he may have had in mind the words of Mal. 4:1, concerning the day of the Lord, "that shall burn as an oven," when all the wicked will "be stubble." The fire of that great day, Malachi continues, would "burn them up" so completely that "neither root nor branch" would remain (ch. 4:1; see also ch. 3:2, 3).
Far from conveying the idea of a fire that burns eternally in which the wicked are endlessly tormented, the Scriptures emphasise the fact that the wicked are to be burned up so completely that nothing will be left of them. The idea of an eternally burning hell fire is foreign to the Inspired Word, as it is to the character of God. The Scriptures state concerning Sodom and Gomorrah that they are "set forth for an example, suffering the vengeance of eternal fire" (Jude 7; cf. 2 Peter 2:6). But the fires that consumed those wicked cities went out long ago; they are not burning today. Yet those cities are given as an "example" of what the fires of the last great day will be like.
In a similar way, Jeremiah predicted that God would kindle a fire in the gates of Jerusalem that would consume even the palaces of the city, and "not be quenched" (Jer. 17:27). This was literally fulfilled a few years later when Nebuchadnezzar took the city, in 586 b.c. (Jer. 52:12, 13; cf. Neh. 1:3). Quite obviously the fire is not burning today. As chaff from a Judean threshing floor was utterly consumed, and nothing but ashes remained, so the wicked will be burned with "unquenchable fire" on the last great day until nothing but their ashes remain (Mal. 4:3). The wages of sin is death (Rom. 6:23), eternal death, not eternal life miraculously preserved by a vengeful God, in the midst of fire that never goes out. The righteous are promised eternal life (Rom. 2:7), and the death of the wicked will be as permanent as the life of the righteous -- Seventh-day Adventist Bible Commentary.