John was speaking to those he called "brood of vipers" in verse 7. Let's look at all of it.
"7 And having seen many of the Pharisees and Sadducees coming about his baptism, he said to them, `Brood of vipers! who did shew you to flee from the coming wrath?
8 bear, therefore, fruits worthy of the reformation,
9 and do not think to say in yourselves, A father we have -- Abraham, for I say to you, that God is able out of these stones to raise children to Abraham,
10 and now also, the axe unto the root of the trees is laid, every tree therefore not bearing good fruit is hewn down, and to fire is cast.
11 `I indeed do baptize you with water to reformation, but he who after me is coming is mightier than I, of whom I am not worthy to bear the sandals, he shall baptize you with the Holy Spirit and with fire,
12 whose fan [is] in his hand, and he will thoroughly cleanse his floor, and will gather his wheat to the storehouse, but the chaff he will burn with fire unquenchable.'" (YLT, bold emphasis mine.)
John was speaking to two sects of the Jews who held political control over the people: the Sadducees controlled the temple and the money, and were seeking favor from the Roman governor so that they would remain in power; and the scribes and Pharisees held themselves out as the most knowledgeable of the law. See The Sadducees and here and Different Sects.
John was not speaking to people who were coming to him to be immersed in water. (Remember that the word "baptism" is an English form of the Greek "baptizo", and instead of being transliterated it should have been translated as "immersed", or "submerged".) Just a few chapters later, Jesus engaged in a conversation with some of these same people and knew they were plotting to kill Him (Matt. 12:14-37). He even calls them the same as had John:
"`Brood of vipers! how are ye able to speak good things -- being evil? for out of the abundance of the heart doth the mouth speak." (YLT)
So, John was not referring to their possible conversion. They were not interested in being converted else they would have asked John to be immersed in the water in which he stood as he spoke to them. He knew them to be evil, because he called them a brood of vipers.
The language all throughout verses 7 -12 is judgment language as can be seen from all OT prophesy. The axe laid to the root of the trees is a picture of strong men about to be cut down. In OT prophesy "trees" were symbolic of powerful men (See Ez. 17:22-24; Ez. 31:3; Jer. 17:7-8; Psa. 1:3, etc). Hewing down a tree was symbolic of cutting down a king, or nation.
Fire was the symbol of God's wrath and judgment. See Deu. 4:24; 32:22-24; Isa. 29:6, Isa. 30:30; Ezek. 38:19-20; Nah. 1:5-6, etc. Anytime we see the word "fire" in the scriptures we should automatically think "judgment" language.
John was telling these evil, wicked people that they were in danger of being burned up in God's judgment. Therefore, the immersion he spoke of "with the Holy Spirit and with fire" was immersion into God's judgment. Immersing someone or something does not always apply to salvation. This was a judgment prophesied by John upon those who plotted to kill the Messiah.
It should not be applied to immersion into water of those who want to be saved.
The NT is filled with judgment language straight out of the OT, and had a great impact on the Jews who knew it from all OT prophesy. For more study on the judgment language of the OT see my posts, "It's Not The End of The World, Part III-Judgment Language in the Old Testament", and Part IV - Judgment Language in Both the Old and New Testament" at here.