work out your own salvation with fear and trembling
If one were truly saved by merely believing in Jesus it's nonesensical for the apostle to instruct us to work out our salvation in fear and trembling. For if we were saved at once, instantly, and for all time we needn't fear, nor would we be capable of having fear, nor would we have need of hope. Jesus' death and resurrect was not the end of hope, but the BEGINNING of hope. Salvation is not acquired once, instantly in this life. It has at least one sure prerequisite: death. So while we are alive we need work out our salvation in hope, trust, hope to overcome the fear of unceatainly, and trust to overcome the trembling reflecting on the awe of God. Salvation cannot be acquired or earned. It, like grace, is a gift from God. Believing in god in humility is but the first step.