Note the whole text of Matt 3:11 -
I baptize you with water for repentance, but after me will come One
more powerful than I, whose sandals I am not worthy to carry. He will
baptize you with the Holy Spirit and with fire.
First note that John makes a deliberate contrast between the opposites of fire and water by saying that John baptises with water but Jesus would baptise with fire symbolising the Holy Spirit. (More on this shortly)
The work and function of the Holy Spirit is frequently associated with the metaphor of fire. Here is a sample:
- Matt 3:11, He will baptize you with the Holy Spirit and with fire.
- Luke 3:16, He will baptize you with the Holy Spirit and fire.
- Acts 2:3, 4, They saw tongues like flames of fire that separated and came to rest on each of them. And they were all filled with the Holy Spirit and began to speak in other tongues as the Spirit enabled them.
- Rom 12:11, Be on fire with the Spirit.
- 1 Thess 5:19, Do not put out the Spirit's fire. (INT)
- Heb 12:29, God is a consuming fire.
- Rev 4:5 where the Spirit is symbolised by torches of fire.
- Jodges 15:14, (about Samson) Then the Spirit of the LORD rushed upon him, and the ropes that were on his arms became as flax that has caught fire,
At the same time, fire, and the context of Matt 3:11 suggests some kind of Judgement. This is consistent with that in the OT in places such as:
- Isa 4:4, he will cleanse the bloodstains from Jerusalem by a spirit of judgment and a spirit of fire.
- Isa 33:11, You conceive chaff, you give birth to straw; your breath is a fire that consumes you.
This judgement fire was to separate (the basic meaning of judgement) the worthwhile material from the waste - good material would be survive the fire and waste burned up. The above texts all have their origin in Mal 3:1-3 about God being a refiner's fire that is used to refine silver; see also Num 31:23. This was the action of Jesus and the Holy Spirit and God more generally that they act as a fire to separate and make evident, or distinguish the righteous from the wicked.