I indeed baptize 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 baptize 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.
To come after someone is a Greek expression that indicates what does to be disciples mean. Here Jesus – who will be baptized by John – is presented by the prophet as his disciple. Immediately afterwards John himself says that the one who will come after him would have been mightier than him and that he would not have been worthy of bearing his shoes. This last action is the simplest task a slave can accomplish. In Jesus’ time, people expected that a rabbi’s disciple should act against the master as a slave. However, the gesture of bearing the shoes of the rabbi was too much even for a slave. This to give an idea of how John considered the Messiah.
John’s baptism was of a symbolic nature and, after the arrival of the mightier, the John’s baptism would have been overtaken by baptism in the Holy Spirit and fire. The prophets of the Old Testament had announced a purifying effusion of the Spirit of God that would have happened in the Messianic age (Is 32:15; 44:3; Ez 36:26-27; 39:29; Joel 2:28-29 ). Purification through fire was also a messianic hope (Is 4:4; Zac 13:9; Mal 3:2). According to the evangelist, the Messiah would bring the people of God to his original holiness that John’s baptism simply prefigured.
This would also have happened through the judgment that the Messiah would have made. Judging activity is imagined here as the separation of wheat from chaff. The chaff will be burned with unquenchable fire. These words should have incited fear and submission to those who wanted to be baptized by him. Jesus will also use similar words (Matthew 13:30) and he will even speak of eternal fire, referring to the fate of the wicked (Matthew 25:41).
However, fire is also an image of the Spirit, an aspect of his activity, the one that gives us the energy and vitality to live a life that is up to the task that has been entrusted to us. Of course the purification aspect of the baptism remains valid, because there is no person who has succeeded in life without suffering! (Comment based on T. France, The Gospel According to Matthew, Eerdmans 1985)