Scripture teaches that those who have trusted Christ as Savior are "in Christ". This has a literal meaning, and when it's understood as literal truth, then you can begin to understand why salvation is eternal. It's because our spirit is made one with Him through the work of the Holy Spirit. We are already "sanctified" (set apart) by the body of Christ.
When we are placed "in Christ", our spirit in Him cannot be corrupted with sin. Sin is not possible in our spirit. If it has no sin, it cannot die. Jesus cannot be corrupted with sin, if we are "in Him", we can't either! This is salvation, and it is eternal from the moment we believe the truth, which is, Christ died, was buried, and rose again for us that through faith in Him and not of ourselves, we are given the gift of life.
2 Corinthians 5:21
For he hath made him to be sin for us, who knew no sin; that we might be made the righteousness of God in him.
Hebrews 10:10
By the which will we are sanctified through the offering of the body of Jesus Christ once for all.
Hebrews 10:14
For by one offering he hath perfected for ever them that are sanctified.
Colossians 1:12-14
Giving thanks unto the Father, which hath made us meet to be partakers of the inheritance of the saints in light: Who hath delivered us from the power of darkness, and hath translated us into the kingdom of his dear Son: In whom we have redemption through his blood, even the forgiveness of sins: