I indeed baptize you with water; but One mightier than I cometh. To refute this growing conviction that he was the Messiah, John tells the people plainly tidal Another far greater than he was coming.
He, John, certainly washed (baptized) those who came to him, but his washing was merely symbolical—it could not purify them; his work had been to stir them up to repentance, to arouse them to change their lives.
But the One who was coming, before whom he (John) was unworthy to stand and perform the humblest menial office, that great One should baptize too, but his baptism would be a very different thing. He shall baptize you with the Holy Ghost.
There was, indeed, a difference between John's baptism and the baptism of the Messiah who was to come after him. John could do no more with his words and symbol baptism than rouse the people to struggle after repentance and a change of heart and life, while Messiah would furnish to men the influence from above, that was really needed in order to purity of heart and life.
He would procure and pour out the influence of the Divine Spirit. And with fire. Not with punitive fire, which interpretation would be quite alien from the context here. Those expositors who have adopted this meaning of the fire here have been most likely influenced by the mention of the unquenchable fire in the next sentence.
The fire which was to enter into Messiah's baptism was rather the flame of purification. So we read of the coal of fire taken from off the altar and laid on the mouth of Isaiah the prophet (Isaiah 6:6, Isaiah 6:7).
"With fire," writes Bishop Wordsworth, "to purify, illumine, transform, inflame with holy fervor and zeal, and carry upward, as Elijah was carried up to heaven in a chariot of fire."