How do we know that we cannot fall away from grace?

The assurance of perseverance for true believers stems from the doctrine of regeneration and the unchanging righteousness of Christ. In 1 John 3:9, it states that those born of God do not sin, meaning they do not commit the 'sin unto death' or apostatize. This is because Christ's seed remains in them, securing their faith until the end. Their continuing faith in Christ is evidence of their election and the sovereign grace that keeps them rooted in salvation. Therefore, a genuine believer will ultimately persevere, not because of their strength, but because God keeps them.