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

The doctrine of 'once saved, always saved' rests on the assurance that God's salvation is effective and permanent. As Rex Bartley states, if God saves us, He saves us forever, as indicated in John 10:28, where Jesus asserts, 'they shall never perish.' This doctrine is reinforced by Psalm 138:8 which declares, 'The Lord will perfect that which concerneth me.' The eternal security of the believer is rooted in God's immutable character and His power to keep us in His hand, ensuring that nothing can separate us from Him, as articulated in Romans 8:38-39.