Nowhere does Paul suggest that justification is in stages. It is always presented to us as a once-for-all declaration, a benefit received through faith alone. Sanctification is inaugurated. Sanctification is progressive. Sanctification is to be consummated but our justification is complete. It is finished.
Δικαιωθέντες οὖν ἐκ πίστεως εἰρήνην ἔχομεν πρὸς τὸν θεὸν διὰ τοῦ κυρίου ἡμῶν Ἰησοῦ Χριστοῦ (Rom 5:1)
For a series of philosophical, theological, and practical reasons the medieval church came gradually to think that our justification, i.e., our acceptance with as righteous God is progressive. What the confessional Reformed and Lutheran churches call sanctification, i.e., our gradual conformity to Christ, the medieval church came to think of as justification. This doctrine of progressive justification became the dogma of the Roman communion at the Council of Trent (session 6, 1547). The confessional Protestants, however, rejected both the medieval consensus and Roman dogma on the authority of God’s Word (sola Scriptura). The Protestants, as distinct from both the Anabaptists and Rome, confessed that justification is definitive and sanctification is progressive, that they are distinct (but related) benefits of God’s unconditional favor (sola gratia) received through faith alone (sola fide). The Protestant churches confessed that the moment one believes one is declared righteous on the basis of the imputation (crediting, reckoning) of Jesus’ perfect righteousness (meritum condignum) to the believer, received through faith alone.
In the Augsburg Confession (1530), art. 4, the Lutherans confessed:
Also they teach that men cannot be justified before God by their own strength, merits, or works, but are freely justified for 2]Christ’s sake, through faith, when they believe that they are received into favor, and that their sins are forgiven for Christ’s sake, who, by His death, has made satisfaction for our sins This faith God imputes for righteousness in His sight. Rom. 3 and 4.
In the Heidelberg Catechism (1563), the Reformed confessed:
21. What is true faith?
True faith is not only a certain knowledge whereby I hold for truth all that God has revealed to us in His Word; but also a hearty trust, which the Holy Spirit works in me by the Gospel, that not only to others, but to me also, forgiveness of sins, everlasting righteousness and salvation are freely given by God, merely of grace, only for the sake of Christ’s merits.
56. What do you believe concerning the “forgiveness of sins”?
That God, for the sake of Christ’s satisfaction, will no more remember my sins, nor the sinful nature with which I have to struggle all my life long; but graciously imputes to me the righteousness of Christ, that I may nevermore come into condemnation.
60. How are you righteous before God?
Only by true faith in Jesus Christ; that is, although my conscience accuse me, that I have grievously sinned against all the commandments of God, and have never kept any of them, and am still prone always to all evil; yet God without any merit of mine, of mere grace, grants and imputes to me the perfect satisfaction, righteousness, and holiness of Christ, as if I had never committed nor had any sin, and had myself accomplished all the obedience which Christ has fulfilled for me; if only I accept such benefit with a believing heart.
One finds precisely the same doctrine in the Belgic Confession (art. 22–24). Article 24 even adds, “for by faith in Christ we are justified, even before we do good works” [emphasis added] to make it perfectly clear that sanctification is the fruit and evidence of our justification and salvation and not the ground or instrument. Westminster Confession ch. 11 explicitly says that we are now presently and completely justified before God by grace alone, through faith alone.