One modern denomination in the U.S. is the “Fire-Baptized Holiness Church”; many other Christians also happily claim to be “baptized in the Holy Ghost and fire.” We know and appreciate, of course, what they mean; they mean holiness, and holiness is essential. But is that what John the Baptist means by “fire baptism” in this passage? Fire is sometimes used as a symbol of God’s consuming holiness or of purifying trials in the Bible; but when fire is conjoined with the image of baptism in the New Testament, it has to do not with mere purification of the individual, but with purifying the whole world by judgment. (Judgment is the most common symbolic application fire in the Bible.) Rather than cross-referencing to other passages that use the image of fire in different ways, we ought to examine what the “baptized in fire” text means in its own context. We ought to use the passage itself before jumping to a concordance.
The context is a call to repentance, and much of the audience promised this fire baptism was unwilling to repent. John the Baptist was immersing people in water as a sign of their repentance and preparation for the coming Kingdom of God (Matt. 3:2, 6). (Jewish people used baptism when non-Jews would convert to Judaism, but John demanded that even religious Jewish people come to God on the same terms on which Gentiles should; cf. 3:9.) John warned the Pharisees about God’s coming wrath (3:7), and that unless they bore fruit (3:8) God’s ax of judgment would cast them into the fire (3:10; cf. 12:33). Fruitless trees were worthless except for fuel. But chaff was barely even useful as fuel (it burned quickly), yet the chaff of which John spoke would be burned with “unquenchable”—eternal—”fire” (3:12).
In the verses just before and just after our verse, “fire” is hellfire (3:10, 12). When John the Baptist speaks of a baptism in fire, he uses an image of judgment that follows through the whole paragraph. Remember that John’s hearers here are not repentant people (3:7). The Messiah is coming to give his audience a twofold baptism, and different members of his audience would experience different parts of it. Some may repent, be gathered into the barn and receive the Spirit. The unrepentant, however, would be chaff, trees cut down, and would receive the fire!