Our only hope for living the radical demands of the Christian life is that God is totally for us now and forever. Therefore, God has not ordained that living the Christian life should be the basis of our hope that God is for us. That basis is the death and righteousness of Christ, counted as ours through faith alone. On the cross Christ endured for us all the punishment required of us because of our sin. And in order that God, as our Father, might be completely for us and not against us forever, Christ has performed for us, in his perfect obedience to God, all that God required of us as the ground of his being totally for us forever.
This punishment and this obedience are completed and past. They can never change. Our union with Christ and the enjoyment of these benefits is secure forever. Through faith alone, God establishes our union with Christ. This union will never fail, because in Christ God is for us as an omnipotent Father who sustains our faith, and works all things together for our everlasting good. The one and only instrument through which God preserves our union with Christ is faith in Christ—the purely receiving act of the soul.
Piper, John. The Future of Justification: A Response to N. T. Wright. (United States: Crossway Books, 2007), 184