Justification is the act in which God declares believing sinners righteous on the basis of Jesus’ death and resurrection (Rom. 5:9–10). Sanctification is both the act by which God sets apart believing sinners to be his holy people in thought, word and deed (1 Cor. 1:30), called definitive sanctification, as well as the process by which God conforms those set-apart people to the image of his Son Jesus (Eph. 5:26), called progressive sanctification. Justification is by faith and not by works (Rom. 3:28). From this fact Protestants use the language of justificatio sola fide: justification by faith alone. But they often don’t proceed to speak of sancificatio sola fide: sanctification by faith alone. The fact is, our sanctification is by faith alone no less than our justification is. By faith alone in Jesus Christ we appropriate his righteousness by which God counts us righteous. He doesn’t count us righteous because we perform righteousness but because we trust in the righteousness Jesus performed. In the same way, God doesn’t make us righteous by asking us to pull up our spiritual shirt sleeves and get to work now that we’re justified so we can win our our (existential) righteousness. No, he tells us to vest our full faith in Jesus Christ to work his righteousness in us as we trust in him and obey (Rom. 6:1–11).
Of course, good works are no less necessary in relation to sanctification than they are to justification. The faith that justifies and sanctifies (it is the same faith) is never alone. The faith that justifies and sanctifies necessarily produces good works and righteousness. The great comfort is that the God who justifies is equally the God who sanctifies. This is why when we stand before God he will judge us according to our works (Lk. 12:48; Rom. 2:6f.): that is to say, he’ll judge us by the essential effects of our faith. To judge by good works is to judge by faith alone, because good works are the effect of God’s work in our lives activated by faith alone. Good works are the “side B” of faith. Good works are the visible faith corresponding to the invisible internal faith (Lk. 5:20).
The language of some New Perspective theologians, “You get into the covenant by grace and stay in by works,” is precisely wrong. You “get in” by a faith that works by love and you stay in by the same faith that works by love (Gal. 5:6). It’s the faith that (instrumentally) gets you in and keeps you in. Oddly, some of the same people who want to sever all works from justifying faith say little about faith alone in sanctifying. They want justificatio sola fide but not sancificatio sola fide. They want a justification that doesn’t necessarily produce good works and a sanctification that doesn’t everywhere and always look to Jesus alone for all it gets. They are antinomians in justification and moralists in sanctification. The preacher that browbeats the congregation to start obeying but never points them to the Jesus to whom they’re united by faith as the only way they can obey (Rom. 5:1–20) is no better than the preacher who soothes sinners with the assurance that if they simply “accept Jesus” they never need submit to him and obey (Mt. 16:24–27). Moralism and antinomianism are equally damnable.
Sinners throw themselves with reckless abandon on Jesus and are justified by that simple, wholehearted faith. Christians throw themselves with reckless abandon on Jesus and are sanctified as they persevere in simple trust in his work in their lives.
The Christian life from beginning to end is by faith alone, a faith that, because it’s God’s work and not man’s, issues in good works.