Salvation as a theological term means to be saved from eternal condemnation and the wrath of God through putting one's faith in Jesus Christ as their Lord and Savior, the only One who is capable and sufficient enough to pay for the person's sins. Scripture says that salvation in God through Jesus Christ is an act of grace through faith, and it is not of works so that no person can ever boast about it (Ephesians 2:8-9). As a point of contention between Catholics and Protestants, the evidence of that salvation is usually through the person doing good works that have been prepared for that person by God (Ephesians 2:10). Entrance into this salvation is through the Sinner's Prayer that the person must recite in faith in order to be saved when a person recognizes their sinfulness before a holy God and their need to be saved. In the Left Behind series, one is immediately saved if they sincerely believe, and those who are saved, as evidence of their salvation, possess the "seal of God" on their forehead which is visible to other believers. The seal of God started to appear on believers just after the Wrath Of The Lamb earthquake, and it served as a means for believers to identify immediately those among themselves.
The Sacrament's Role in Salvation
The Left Behind series' emphasis on faith and the exigent circumstances of the Tribulation resulted in the authors eliding over the importance of sacramental activities in Christian life (with the exception of marriage). In the series, the efficacy of baptism in salvation is not a theological issue because few characters mention "baptism" and no one gets baptized (although it is said that Peter Mathews baptized Caryn Litewski before the Rapture). In Left Behind: World at War, some believers do take communion, and the communion wine (called "wine" there) serves as a miraculous antidote for an illness that was spread by Bibles infected by the Global Community.
The Role of Human Will and Predestination
The series does not espouse a Calvinist/Reformed view on soteriology where it is God's Holy Spirit that grants irresistible grace to receive saving faith. According to this view, one does not become a believer through their own volition but faith itself is an act of God's sovereign determination. It seems that some of the characters, even those who understand the Christian message of salvation enough to encourage others to accept Christ, can resist this grace, such as Hattie Durham before she became a believer in The Mark. While Krystall Carney was protesting the notion that taking Carpathia's mark is irrevocable, she objected by saying that she still had her own free will. Rayford Steele responded that she "apparently not" has the will to accept God's grace anymore.
In Assassins, before the demonic horsemen judgment, in a web letter to believers, Tsion Ben-Judah says a quarter of those left behind had been killed by wars, plagues, and natural disasters, but a third of that population would be killed (of whom would be unbelievers) by the 200 million horses, thus one half of the world's population after the Rapture would remain. He offers the explanation that that judgment serves to winnow down the forces of evil to set the stage for the eventual "final battle between good and evil". Tsion Ben-Judah qualifies his remarks as speculation and by extension the authors of the series, perhaps because the authors do not want the series explicitly be tied to a particular theory of soteriology. Tsion Ben-Judah says that those who would be killed were "incorrigibles whom He, in His omniscience, knows would never have turned to him regardless". The horsemen presumably pick their quarry among the vulnerable unbelievers, so it is not God that chooses their targets for them. Tsion Ben-Judah's explanation would imply that God would render immune those whom He knows from His omniscience that would become believers later on in the Tribulation (in addition to those who are believers with the seal of God). That would imply a Molinistic view of salvation that ostensibly preserves man's agency in salvation and God's omniscience. The scenario that Tsion Ben-Judah describes requires God having "middle knowledge" of the counterfactuals. In this case, God would have the counterfactual knowledge that in the case that those individuals who were killed by the horsemen instead survived the sixth trumpet judgment they would have not come to faith anyway because they would be stubborn or taken the mark of loyalty. This is in contrast to the view that God actively selects those to be saved according to His volition as opposed to passively knowing one's status on the basis of counterfactual knowledge or dispensing grace in accordance to the counterfactual knowledge of those who would be receptive to it. The view that God selects those according to His merciful volition without any reference to any potential merit of the recipients of God's grace is called "unconditional election".
However, the books do subscribe to the perseverance of the saints (once saved, always saved), one of the five points of Calvinism, where God's salvation is taught as being eternal, meaning that once a person is saved and becomes a Christian, they can never be lost. Tsion Ben-Judah mentions that one who faces martyrdom in the sight of the guillotine for refusing the "mark of loyalty" would be unable to deny Jesus under duress. (The disciple Peter was able to deny Jesus three times the night before Jesus was crucified.) He says resistance to the mark of loyalty is a supernatural act as "God would miraculously overcome our evil, sinful flesh and gives us the grace and courage to make the right decision in spite of ourselves." Nevertheless, in The Remnant, about half of the dissident Muslims were willing to die for resisting Carpathianism even before they were exhorted to have faith in Christ by the angel Christopher.
God's salvation is offered freely throughout the entire period of the Tribulation to those who have not taken the "mark of loyalty" and have not worshiped the image of Nicolae Carpathia, the Antichrist, with its cutoff being the Lord's Second Coming at the end of the Tribulation. In the series, it is God's will for everyone to be saved; salvation is not limited to whom He predestines to be saved. In a speech in The Remnant, Tsion Ben-Judah cites 2 Peter 3:9 that God is not willing that any should perish but instead come to repentance. Also, in that speech, Tsion Ben-Judah says that "salvation is a personal decision" where one admits to God and themselves that they are a sinner. In Armageddon, Tsion Ben-Judah, however, warns that there are those in the latter half of the Tribulation who have not received the mark of the beast whose hearts have hardened that they cannot choose to believe in Christ even if they wanted to. In the Millennial Kingdom, a "natural" who has God's salvation in Jesus Christ before their 100th birthday enjoys the benefit of living until the end of the period with exponentially slowed aging and entering into the New Jerusalem at the end.
The Role of God's Grace
In Apollyon, when Hattie Durham confessed to Rayford Steele that she helped frame Amanda White as an agent of the Global Community, she said that she has done many things that made her unworthy of God's love and forgiveness. Hattie was not able to believe that God could and should forgive her. Rayford does confirm that no one deserves God's forgiveness, but Hattie points out that Rayford still accepted God's forgiveness anyway. Rayford points out that Hattie's rejection of God forgiveness is an act of pride on her part because she adamantly declares that she is unworthy and unwillingly to accept God's forgiveness. Rayford crystallizes the discrepancy between the intrinsic lack of merit of humans and God's profound offer of forgiveness and salvation with the term "grace". Hattie says that there is too much of a gap between "what may be true (presumably God's forgiveness and grace) and what should be true (presumably Hattie's condemnation)" and Rayford introduces another term "inequity" to describe it.
Using the term "inequity" is odd, since it is an admission that God's grace ostensibly violates some standard of justice. Christian theologians typically appeal to Jesus' atonement as the means of reconciling this since Christ's blood somehow appeases God's sense of justice by paying for mankind's sins. Before her martyrdom, Hattie Durham proclaimed to the assembled in Jerusalem before Leon Fortunato that Jesus is the one mediator between God and man (1 Timothy 2:5). However, one can understand the Parable of the Prodigal Son as illustrating that no atonement is necessary for forgiveness since the father did not require his son to do anything after his son returned home and just forgave him unconditionally.
In other series with a premillenialist eschatology, God can even forgive people who not only sinned against God, but led others to perdition. Reverend Matthew Turner in A Thief in the Night series refuses to accept God's forgiveness (and also refuses the mark of the beast) because he had led many astray by preaching "godless humanism" to his parishioners when he knew the gospel. In contrast, in the Left Behind series, Pastor Bruce Barnes probably did preach the gospel, but did not believe in it since he was not raptured. Also, Marshall Jameson owned a Christian radio station that presumably saved people and edified people's faith, but he was not raptured. That would not be so bad since the gospel is preached regardless of their motivations (Philippians 1:15-18). In the Christ Clone trilogy, Decker Hawthorne even come up with the suggestion of marking people with "666" but was forgiven by God as he was dying and was resurrected during the start of the Millennium. While he was involved in the administration of the Antichrist, through coincidence, he did not receive the mark of the beast.
God's grace cannot be merited through one's character and works (Romans 11:6). When Patrick Rose objected that having the mark of loyalty prevented him from going to heaven to see his raptured wife again saying that he has been trying to help people all his life and God should take that into account, Judd Thompson Jr. answered that "it is not about doing good things". If God's grace and the reward of eternal life are truly a gift, then it cannot be merited through one's deeds or personal character. In order for God to be sovereign, He has to be able to deprive salvation even to those who would be deemed "good" through adherence through some code of conduct or by garnering the approbation of one's peers. For example, Dr. Patrick Rose was a gracious host to Judd and treated his friend, Lionel Washington, whose arm was crushed by a rock slide, and he did not report Judd and Lionel to the Global Community. That applies even if one rejects the notion of original sin or eternal torment. The Protestant reformers promote the concept of imputed righteousness where the beneficiaries of God's grace are credited with the righteousness of Christ while human nature, corrupted by original sin, cannot even desire to attain such righteousness without the influence of the Holy Spirit to give one faith.
In Kingdom Come, Rayford Steele said that the only benefit of Cameron Williams speaking at Cendrillon Jospin's funeral was to "warn other young people of the consequences of putting off the transaction with Jesus", something that Cendrillon Jospin did not do. Thus, salvation can be seen as performing a transaction such as entrusting one's life to Jesus.