John Piper from the Desiring God website:
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. All the punishment required of us because of our sin, Christ endured for us on the cross. And all the obedience that God required of us, that he, as our Father, might be completely for us and not against us forever, Christ has performed for us in his perfect obedience to God.
This punishment and this obedience (not all obedience) is completed and past. It 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.
The Place of Our Good Works in God’s Purposes
Our own works of love do not create or increase God’s being for us as a Father committed to bringing us everlasting joy in his presence. That fatherly commitment to be for us in this way was established once for all through faith and union with God’s Son. In his Son, the perfection and punishment required of us are past and unchangeable. They were performed by Christ in his obedience and death. They cannot be changed or increased in sufficiency or worth.
Our relationship with God is with One who has become for us as an omnipotent Father committed to working all things together for our everlasting enjoyment of him. This relationship was established at the point of our justification when God removed his judicial wrath from us, and imputed the obedience of his Son to us, and counted us as righteous in Christ, and forgave all our sins because he had punished them in the death of Jesus.
Therefore, the function of our own obedience, flowing from faith--that is, our own good works produced as the fruit of the Holy Spirit—is to make visible the worth of Christ and the worth of his work as our substitute-punishment and substitute-righteousness. God’s purpose in the universe is not only to be infinitely worthy but to be displayed as infinitely worthy. Our works of love, flowing from faith, are the way Christ-embracing faith shows the value of what it has embraced. The sacrifices of love for the good of others show the all-satisfying worth of Christ as the One whose blood and righteousness establishes the fact that God is for us forever.
All the benefits of Christ—all the blessings that flow from God being for us and not against us—rest on the redeeming work of Christ as our Substitute. If God is for us, who can be against us? With this confidence—that God is our omnipotent Father and is committed to working all things together for our everlasting joy in him—we will love others. God has so designed and ordered things that invisible faith, which embraces Christ as infinitely worthy, gives rise to acts of love that make the worth of Christ visible. Thus, our sacrifices of love do not have any hand in establishing the fact that God is completely for us, now and forever. It’s the reverse: The fact that God is for us establishes our sacrifices of love. If he were not totally for us, we would not persevere in faith and would not therefore be able to make sacrifices of love.
Our mindset toward our own good works must always be: These works depend on God being totally for us. That’s what the blood and righteousness of Christ have secured and guaranteed forever. Therefore, we must resist every tendency to think of our works as establishing or securing the fact that God is for us forever. It is always the other way around. Because he is for us, he sustains our faith. And through that faith-sustaining work, the Holy Spirit bears the fruit of love.
Avoiding the Double Tragedy
There would be a double tragedy in thinking of our works of love as securing the fact that God is completely for us. Not only would we obscure the very reason these works exist—namely, to display the beauty and worth of Christ, whose blood and righteousness is the only and all-sufficient guarantee that God is for us—but we would also undermine the very thing that makes the works of love possible—namely, the assurance that God is totally for us, from which flows the freedom and courage to make the sacrifices of love.
Our obedience does not add to the perfection and beauty and all-sufficiency of Christ’s obedience in securing the reality that God is for us; it displays that perfection and beauty and all-sufficiency. Our works of love are as necessary as God’s purpose to glorify himself. That is, they are necessary because God is righteous—he has an eternal and unwavering commitment to do the ultimately right thing: to make the infinite value of his Son visible in the world.