John said that Jesus would Baptize with the Holy Ghost and with fire. The Holy Ghost would be poured out on believing Israel just as the prophets had predicted but unbelievers would be “burned up.” The “fire” is judgement.
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. Matt. 3:11-12
And now also the axe is laid unto the root of the trees: therefore every tree which bringeth not forth good fruit is hewn down, and cast into the fire. Matt. 3:10, Matt. 7:19, Matt.13:41-42,
If a man abide not in me, he is cast forth as a branch, and is withered; and men gather them, and cast them into the fire, and they are burned. John 15:6
Which is a manifest token of the righteous judgment of God, that ye may be counted worthy of the kingdom of God, for which ye also suffer: Seeing it is a righteous thing with God to recompense tribulation to them that trouble you; And to you who are troubled rest with us, when the Lord Jesus shall be revealed from heaven with his mighty angels, In flaming fire taking vengeance on them that know not God, and that obey not the gospel of our Lord Jesus Christ: Who shall be punished with everlasting destruction [by “flaming fire”] from the presence of the Lord, and from the glory of his power; When he shall come to be glorified in his saints, and to be admired in all them that believe (because our testimony among you was believed) in that day. 2Thess. 1:5-10