John Piper writes of the centrality of the cross in perseverance, the fact that the doctrine of justification by faith in Christ alone (the Gospel) is what fuels and sustains (and begins) us in the call to endure to the end:
"This is what we pray for, and this is what we trust in as we take up the biblical command to endure to the end. We trust in the New Covenant promises of sustaining, enabling grace that were obtained for us infallibly and irrevocably by Jesus Christ in his death and resurrection. Therefore our fight and our race and endurance is radically God-centered, Christ-exalting, Spirit-dependent, promise-supported life. It is not a 'just do it' ethic. It is not a moral self-improvement program. It is not a 'Judeo-Christian ethic' shared by a vaguely spiritual culture with a fading biblical memory. it is a deeply cross-enabling life that knows the Christ of the Bible as the Son of God who was crucified first as our substitute and then as our model of endurance."
— John Piper, The Roots of Endurance, 29
In other words, our prior understanding of the Gospel (which is itself the result of God's prior activity in us) fuels our obedience. We don't do good things and strive to endure in order to become justified in God's sight. Quite the opposite: because we are justified by faith in the finished work of Christ, we naturally grow in grace and are sustained to the end in good works.
