Our assurance of eternal salvation is based solely on our having trusted in Christ as Savior.
As believers we have been saved by grace through faith, but we are also being saved by grace as we grow in Christ and deal with sin in our lives.
It is impossible for us to work to earn our salvation because salvation is totally of grace and nothing of human effort.
While we were sinners deserving nothing but God’s wrath, we have been made trophies of God’s grace.