The gospel brings us a salvation that is eternally secure. God preserves us. Eternal security is God’s assurance of our salvation. God is absolutely certain of the security of our salvation because He is the one who is securing it. God saves, seals, and secures.
Assurance is our confidence in God saving us and securing us.
The ultimate secret to assurance of salvation is to base your present assurance of salvation on exactly what you based your past experience of salvation – FAITH in CHRIST ALONE, not in yourself.