The truth of preservation is implied when the scriptures describe the saved as being children of God. Hence, they are in vital relationship with God in the same sense in which a son has vital relationships with his natural father. This relationship can never be destroyed. That is, a man can never cease to be his father's son. However, the fellowship between a father and son can be greatly diminished through the son's disobedience. This is also the case between God and His children.
The scriptures directly assert that all saved persons are the current possessors of eternal life (Jn 3:36, Jn 5:24, Jn 6:47, Jn 6:54, 1Jn 5:11) and that this life can never be lost (Jn 6:37-39, Jn 10:28, Rom 8:35-39, 1Pet 1:3-5). Indeed, life which can be lost cannot properly be called eternal.
The Lord's people are said to be predestined to glorification (Rom 8:28-30, Eph 1:3-6), and a predestined event is irreversible by definition. Romans 8:29 asserts that all who are called and justified are also predestined to glorification. It follows then that any who fall short of glorification must never have been truly called and justified.
Man can be condemned to hell only after a valid charge has been made against him. This cannot be the case for God's children (Rom 8:33) since they are sanctified once for all (Heb 10:10) and are forever perfected (Heb 10:14).
Were eternal life secured by man's power, it would doubtlessly be lost. However, the scriptures teach eternal life is secured by the infinite power of God (Jn 10:27-29, 1Cor 1:8, Philip 1:6, 1Thes 5:23, 1Pet 1:5).
The doctrine of preservation is not intended to teach that saved persons can sin with impunity. The scriptures teach God will bring corrective chastisement against all of His disobedient children (Ps 93:12-13, 1Cor 11:32, Heb 12:6-8, Rev 3:19). Accordingly, scriptures teach the power of God's Spirit is ever at work within His children bringing forth the fruits of righteousness (Philip 2:13, 1Thes 4:9, 2Thes 3:3).
Though the Spirit of God moves His people both to will and to do His good pleasure (Philip 2:13), both scriptures and experience teach this Divine influence is not of such degree to bring forth complete sinlessness. Therefore, the good works performed by saved persons cannot account for the preservation of their eternal salvation. Salvation is both obtained and maintained by the grace of God. The scriptures often associate good works with eternal salvation, but salvation is not the result of works; rather, works are the result of salvation (Mt 12:33-35, Jn 8:43-47, Jn 10:25-29, Jn 15:16-19, Acts 11:18, Acts 13:48, Acts 16:14, 1Cor 1:22-24, 1Cor 1:30-31, Gal 5:22-24, Eph 1:4-5, Eph 2:10, 1Thes 1:4-5, 2Thes 2:13-14, 1Pet 2:9).