“For, since Christ is not only man, but God and man in one undivided person, he was as little subject to the law, being Lord of the law, as it would have been necessary for him to suffer and die for his own person. His obedience, therefore, not only in suffering and dying, but in his being voluntarily put under the law in our stead, and fulfilling it with such obedience, is imputed unto us for righteousness; so that, for the sake of this perfect obedience, which he rendered unto his heavenly Father for us, in both doing and suffering, in his life and death, God forgives us our sins, accounts us as righteous and just, and saves us eternally. This righteousness is offered unto us through the Gospel and in the Sacraments, by the Holy Spirit; and through faith it is applied, appropriated, and embraced; hence believers derive reconciliation with God, remission of sins, the grace of God, adoption as children, and the inheritance of eternal life.
Accordingly, the word to justify here signifies to declare just and absolved from sins, and to account as released from the eternal punishment of sins, for the sake of the righteousness of Christ, which is imputed by God to faith, Phil. 3:9. And this usage or import of that word, is common in the holy Scriptures of the Old and New Testaments. Prov. 17:15: ‘He that justifieth the wicked, and he that condemneth the just, even they both are abomination to the Lord’
(Formula of Concord, Full Declaration, III, Henkel p. 633).