Baptism by fire means immersion into the Lake of Fire. It is the second, and therefore eternal, death for the incorrigibly wicked, who have willingly and persistently rejected the salvation of God the Father through Jesus Christ. All who have blasphemed the Holy Spirit and committed the unpardonable sin will be destroyed.

Scripture References:
Matthew 3:11-12;   Matthew 12:31-32;   Revelation 20:14-15;   Revelation 21:8;   II Peter 3:10-12;   Malachi 4:1-3;