Fire is a form of combustion or burning, in which substances combine chemically with oxygen from the air and typically give out bright light, heat, and smoke.
Fire is used in the Bible as both an agent of destruction and a form of purification. In the book of Genesis, the cities of Sodom and Gomorrah were destroyed by fire and brimstone raining from the sky. In the book of Exodus, the angel of God appeared to Moses in the form of fire that was burning a nearby bush without consuming it. Scripture speaks of God Himself being "a consuming fire", and Jesus Christ was spoken of as baptizing people with the Holy Spirit and with fire.
Fire appears in the Left Behind series in the form of natural occurrences as well as supernatural ones. The Two Witnesses that prophesied during the first half of the Tribulation were able to breathe fire that consumed whoever would try to attack them. Leon Fortunato would be able to call down fire to consume whoever would oppose him and his master Nicolae Carpathia. The first Trumpet Judgment would cause fire and hail mingled with blood to fall from the sky and burn up a third of the earth's vegetation and all green grass. At the end of the Millennium, God would cause fire to instantly consume The Other Light army at The Last Battle. The old earth itself would be consumed by fire that was ignited from the earth's core.