How do we know that once saved, always saved is true?

The doctrine of 'once saved, always saved' finds its roots in the security believers have in their union with Christ. Romans 8:30 illustrates that those whom God predestined, He called, justified, and ultimately glorified. This chain of salvation signifies that if one is genuinely saved, they cannot fall away, as salvation is based on Christ's work and not our own. When we place our faith in Christ, we are declared righteous and given eternal life, protected by the grace of God. Our security does not depend on our actions but on Christ’s completed work on the cross, assuring us that our salvation is secure.