[Please note: Much of the following material is "plagarized" and mined from various places without reference to the source.
It is provided simply because it is helpful for understanding this issue. ]
Simul Iustus et Peccator
To one who does not work but trusts him who justifies the ungodly, his faith is reckoned as righteousness.
How is the Christian to see himself in this world? "Simul iustus et peccator" - "At the same time righteous and a sinner". Justification is forensic. In Christ, we are declared, counted or reckoned to be righteous when God imputes the righteousness of Christ (an "alien righteousness") to our account. Christ's righteousness ascribed to the redeemed individual without their personal merit. We are declared righteous in Christ, it is imputed to us -- it is counted as ours ... not infused in us. We are counted righteous in God's eyes because of Christ. But this does not make us righteous in ourselves. That will only happen at our glorification when Christ transforms these bodies to be sealed in righteousness. Justifying righteousness is something which always resides in the Person of Christ alone. The imputation of this "alien" righteousness is the only means by which man can be acceptable to God. As long as the Christian lives, he is guilty in himself, but "in Christ" he is righteous and accounted precious.
The Council of Trent itself reveals that Rome considered Luther's simul iustus et peccator to be a most serious threat to the traditional teaching of the Catholic church. The Roman Church contended that “justification” means making a man righteous in his own person. The Catholic reasons, “How can God pronounce a man to be righteous in His sight unless he is actually righteous?” He therefore thinks that a man must be born again and transformed before he can have right standing with God. In this system of thought, a man can have no real assurance of justification, for he can never be sure whether the Holy Spirit has made him righteous enough to be accepted of God.
Righteousness through Christ is called an “alien” righteousness because it did not generate from us. It is not our righteousness; it is his. It is an alien righteousness because it came from without, and now it is in a foreign land. It does not belong here; it is an alien righteousness. In Latin we call it simul iustus et peccator: simul, simultaneously; iustus, just; et, and; peccator, sinful. That is me – simultaneously righteous and sinful. That is my contribution to salvation -- my sin! At the same time that I am a sinner, God sees me as righteous because of the blood of Jesus Christ. That is the message of outreach -- it is the message of salvation.
Righteousness comes in two ways: coram deo (righteousness before God) and coram hominibus (before man). Instead of a development in righteousness based in the person, or an infusion of merit from the saints, a person is judged righteous before God because of the works of Christ. But,absent the perspective of God and the righteousness of Christ, based on one’s own merit—a Christian still looks like a sinner. The declaration involves God imputing to the believer's "balance sheet" or account the alien righteousness of Christ. The believer is not declared righteous by virtue of his own merit, but on the basis of the merit of Christ. When united to Him, it is justification which becomes the foundation upon which the believer can stand with confidence coram dei. The believer has no cause to fear in the presence of God because of His acquittal. The believer has only and always to look to the finished work of Christ on the Cross and hear God's declaration, "You are accepted." Because of justification the believer does not fear God's rejection because of the sin still present in his/her life. God does not look at the sin in our life except through the work of Christ. This tension is resolved in the Incarnate Christ, crucified and now risen for the life of the world.
Eternal life is Christ dwelling in His righteousness in the soul of the justified person. So eternal life is union with Jesus Christ. And the word for that union with him is faith. The sinners comes to him, rests in him, trusts in him, is one with him, abides with him; and this is life because it never ends. The united soul abides in the Vine eternally. Weakness, sin, proneness to sin never brings separation, but only the Father's pruning, which cements the union even and ever tighter.
the Judge of all the earth declares us "not guilty" when we believe because Christ was pronounced "guilty" for us on the cross. We are not first made righteous, then declared righteous; we are declared righteous by grace through faith in Christ, then made righteous! When we believe, God imputes Christ's righteousness to us 'as if' it were our own. However, it is HIS righteousness, that is why Paul says in Romans 1:17 that there is a righteousness that has been revealed from God, a righteousness not of our own, but a righteousness revealed from God and freely given to those who do not work, but to those who believe. In light of the goodness and graciousness of God who was in Christ reconciling the world to himself, we should daily repent of our own self-righteousness (our works), The words imply a declaration and pronouncement from the divine court of the believer’s right standing with God. “Justification” in itself does not mean a change in the man, but a declaration of how he appears in God’s sight.
Through faith we run to Christ and hold fast to Him, who satisfied the law on our behalf (Romans 10:4; Galatians 3:10-13). In this way we are accounted righteous in the sight of God through faith alone, without doing the works of the law. We are simul iustus et peccator.
Luther recognized that even in a state of regeneration the believer still lives in the world and still in fact does commit acts of sin. There is no attempt to redefine sin to make it anything less than what it is. Rather there is a stark recognition of the dialectic of the Christian's acceptance before God and the fact that he still sins. Luther's phrase to describe this condition was that the state of the Christian between regeneration and ultimate glorification is simul iustus et peccator, at once just (or justified) and sinner. This is not a condition that will ever be transcended in this life. Rather, the believer must always rely on the finished work of Christ for his/her acceptance before God.
Note the helpful definition given by the Westminster Confession
"Those whom, God effectually calls he also freely justifies, not by infusing righteousness into them, but by pardoning their sins, and by accounting and accepting their persons as righteous; not for anything wrought in them or done by them, but for Christ’s sake alone; not by imputing faith itself, the act of believing, or any other evangelical obedience to them as their righteousness, but by imputing the obedience and satisfaction of Christ unto them, they receiving and resting on him and his righteousness by faith, which faith they have not of themselves, it is the gift of God" – WCF Ch 11
WCF Ch 11
"iniquities are forgiven," "sins are covered," "the Lord does not reckon sin against us." Romans 4:5-8
We fully affirm the following with John Knox, Scots Confession 15
"We confess and acknowledge that the law of God is most just, equal, holy, and perfect, commanding those things which, when perfectly done, can give life and bring man to eternal felicity; but our nature is so corrupt, weak, and imperfect, that we are never able perfectly to fulfill the works of the law. Even after we are reborn, if we say that we have no sin, we deceive ourselves and the truth of God is not in us. It is therefore essential for us to lay hold on Christ Jesus, in his righteousness and his atonement, since he is the end and consummation of the Law and since it is by him that we are set at liberty so that the curse of God may not fall upon us, even though we do not fulfill the Law in all points. For as God the Father beholds us in the body of his Son Christ Jesus, he accepts our imperfect obedience as if it were perfect, and covers our works, which are defiled with many stains, with the righteousness of his Son. We do not mean that we are so set at liberty that we owe no obedience to the Law--for we have already acknowledged its place--but we affirm that no man on earth, with the sole exception of Christ Jesus, has given, gives, or shall give in action that obedience to the Law which the Law requires. When we have done all things we must fall down and unfeignedly confess that we are unprofitable servants. Therefore, whoever boasts of the merits of his own works or puts his trust in works of supererogation, boasts of what does not exist, and puts his trust in damnable idolatry."
The “Justification” of Christmas By Charles R. Biggs
Justification By Faith Alone by Jonathan Edwards
The Doctrine of Justification by Zackarius Ursinus
Luther's Commentary on Galatians
Galatians: Paul's Charter of Christian Freedom by Leon Morris
The Everlasting Righteousness by Horatius Bonar
Justification by Francis Turretin
Institutes of Elenctic Theology, 3 vol. set by Francis Turretin