Sin is considered a transgression against God, who created mankind to be holy and faithful unto Him. The Greek word for sin is hamartia (àµaρtίa), which means to "miss the mark". In the epistle to the Romans, Paul taught that sin entered the world through one man, and through sin, death (both spiritual and phyiscal) manifested itself on all people, since all have sinned (Romans 5:12). The book of Hebrews says that there's pleasure in sin, but it's temporary, which makes those who are addicted to sin (or particular types of sin) want to do it more. As Adam is the pattern (typos/τύπος) of the one to come, salvation came through one man, Jesus Christ, just as death reigned through one man (Romans 5:14-17). God worked to redeem mankind from the power of sin by sending His Son Jesus Christ to die for our sins by shedding His blood on the cross, and to rise from the dead three days later. For in Christ's death, believers are united in Christ in His death where one's sinful nature has died along with Christ so one would no longer be a slave to sin (Romans 6:5-7; Galatians 5:24).
In the Left Behind series, God uses His divine judgments throughout the Tribulation to get man to turn away from sin and receive salvation in Him through His Son Jesus before His Second Coming. While many do turn away from sin and become Christians during that time, many more reject the gospel message and instead turn to worshiping Nicolae Carpathia as their god and lord while they continue to practice their sins.
In the Millennial Kingdom, the naturals who reject Jesus as their Lord and Savior instead choose to align themselves with Satan and The Other Light while they live sinful lives. As a result, they go to hell before they are resurrected to face judgment at the Great White Throne and then be cast into the Lake of Fire. Naturals that commit a sin worthy of death could also be terminated before their 100th birthday, as an Other Light member found out when he tried to rape a female glorified in an attempt to impregnate her with a child that would be able to live past 100 years of age without needing to choose Jesus as his Lord and Savior.