When Paul speaks of justification in his letter to the Romans, he clearly builds a judicial or forensic context for the meaning of the word. Specifically, to be justified means to be declared righteous before God on the Judgment day that is coming:
For all who have sinned without the law will also perish without the law, and all who have sinned under the law will be judged by the law. For it is not the hearers of the law who are righteous before God, but the doers of the law who will be justified. For when Gentiles, who do not have the law, by nature do what the law requires, they are a law to themselves, even though they do not have the law. They show that the work of the law is written on their hearts, while their conscience also bears witness, and their conflicting thoughts accuse or even excuse them on that day when, according to my gospel, God judges the secrets of men by Christ Jesus (Romans 2.12-16).
So everyone will be judged and, at that judgment, some will be justified, officially found righteous before God. Thus, we find that to be justified is the antonym of being condemned:
Who shall bring any charge against God’s elect? It is God who justifies. Who is to condemn? (Romans 8.33-34)
So justification is a courtroom act. The judge declares a person to be righteous. But does God justify or condemn only on the Last Day?
Let’s expand a bit on Romans 8.33ff:
Who shall bring any charge against God’s elect? It is God who justifies. Who is to condemn? Christ Jesus is the one who died—more than that, who was raised—who is at the right hand of God, who indeed is interceding for us (Romans 8.33-35).
While God will justify and condemn on the Last Day, there is a sense (and an important one!) in which he has already declared his verdict. Jesus suffered condemnation in history on the first “Good Friday.” And likewise, Jesus experienced God’s verdict over him on Easter Sunday.
No unbelief made him [Abraham] waver concerning the promise of God, but he grew strong in his faith as he gave glory to God, fully convinced that God was able to do what he had promised. That is why his faith was “counted to him as righteousness.” But the words “it was counted to him” were not written for his sake alone, but for ours also. It will be counted to us who believe in him who raised from the dead Jesus our Lord, who was delivered up for our trespasses and raised for our justification (Romans 4.20-25).
Jesus was justified by the Spirit (Romans 8.11; c.f. First Timothy 3.16). God declared Jesus righteous by a public vindication. Jesus was justified by being raised from the dead.
So it would be improper infer that we are not justified until we too are raised from the dead. The justification has already been made in the death and resurrection of Jesus. But to whom does this justification apply? Who shares in the vindication of Jesus? Paul makes it clear that our belief is our vindication even now. God declares us right with him by delivering us from unbelief and all the sins that follow from it.
Do you not know that all of us who have been baptized into Christ Jesus were baptized into his death? We were buried therefore with him by baptism into death, in order that, just as Christ was raised from the dead by the glory of the Father, we too might walk in newness of life.
For if we have been united with him in a death like his, we shall certainly be united with him in a resurrection like his. We know that our old self was crucified with him in order that the body of sin might be brought to nothing, so that we would no longer be enslaved to sin. For one who has died has been justified from sin. Now if we have died with Christ, we believe that we will also live with him. We know that Christ, being raised from the dead, will never die again; death no longer has dominion over him. For the death he died he died to sin, once for all, but the life he lives he lives to God. So you also must consider yourselves dead to sin and alive to God in Christ Jesus.
Let not sin therefore reign in your mortal body, to make you obey its passions. Do not present your members to sin as instruments for unrighteousness, but present yourselves to God as those who have been brought from death to life, and your members to God as instruments for righteousness. For sin will have no dominion over you, since you are not under law but under grace (from Romans 6).
So Jesus’ justification, which was his resurrection, is now ours. He received a condemnation he did not deserve in order to share with us a vindication we do not deserve. But even though we have not been raised from the dead yet, we are still, in common with Jesus new life, raised to a new way of living by the Spirit who raised Jesus from the dead.
This new life is not perfect. We still daily rely on God’s forgiveness, but it is a real break with the state of death and enslavement to sin with which we were cursed. And the most fundamental aspect of this new obedience is faith itself. As goes on to write in Romans 6:
Do you not know that if you present yourselves to anyone as obedient slaves, you are slaves of the one whom you obey, either of sin, which leads to death, or of obedience, which leads to righteousness? But thanks be to God, that you who were once slaves of sin have become obedient from the heart to the standard of teaching to which you were committed, and, having been set free from sin, have become slaves of righteousness (Romans 6.16-18).
Paul is referring to the Romans having come to saving faith. He describes this new faith as having “become obedient fromt he hear to the standard of teaching.” This lines up with the what Paul writes and the opening and closing of Romans:
we have received grace and apostleship to bring about the obedience of faith for the sake of Jesus’ name among all the nations
For I will not venture to speak of anything except what Christ has accomplished through me to bring the Gentiles to obedience—by word and deed, by the power of signs and wonders, by the power of the Spirit of God—so that from Jerusalem and all the way around to Illyricum I have fulfilled the ministry of the gospel of Christ… Now to him who is able to strengthen you according to my gospel and the preaching of Jesus Christ, according to the revelation of the mystery that was kept secret for long ages but has now been disclosed and through the prophetic writings has been made known to all nations, according to the command of the eternal God, to bring about the obedience of faith—to the only wise God be glory forevermore through Jesus Christ! Amen.
There are a couple of things you need to keep in mind:
1. Sin is not only the reason for condemnation but is the penalty of unbelief
Paul makes this clear:
For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men, who by their unrighteousness suppress the truth. For what can be known about God is plain to them, because God has shown it to them. For his invisible attributes, namely, his eternal power and divine nature, have been clearly perceived, ever since the creation of the world,in the things that have been made. So they are without excuse. For although they knew God, they did not honor him as God or give thanks to him, but they became futile in their thinking, and their foolish hearts were darkened. Claiming to be wise, they became fools, and exchanged the glory of the immortal God for images resembling mortal man and birds and animals and creeping things.
Therefore God gave them up in the lusts of their hearts to impurity, to the dishonoring of their bodies among themselves, because they exchanged the truth about God for a lie and worshiped and served the creature rather than the Creator, who is blessed forever! Amen.
For this reason God gave them up to dishonorable passions. For their women exchanged natural relations for those that are contrary to nature; and the men likewise gave up natural relations with women and were consumed with passion for one another, men committing shameless acts with men and receiving in themselves the due penalty for their error.
And since they did not see fit to acknowledge God, God gave them up to a debased mind to do what ought not to be done. They were filled with all manner of unrighteousness, evil, covetousness, malice. They are full of envy, murder, strife, deceit, maliciousness. They are gossips, slanderers, haters of God, insolent, haughty, boastful, inventors of evil, disobedient to parents, foolish, faithless, heartless, ruthless. Though they know God’s righteous decree that those who practice such things deserve to die, they not only do them but give approval to those who practice them.
Thus Paul presents the world of downwardly spiraling sin as a prison world for sinners. They are not only punished for sin, but they are sinners as a punishment for sin. To be given a declarative verdict is, thus, a declaration of freedom from sin. That is why Paul can say so readily in Romans 6 of being “justified from sin.”
2. The primal sin is unbelief so that the gift of faith is also the primal beginning of sanctification.
Faith fundamentally receives God’s grace; unbelief fundamentally spurns God’s blessing in ingratitude: “For although they knew God, they did not honor him as God or give thanks to him” (Romans 1.21). Paul draws this out by portraying Abraham’s faith in Romans 4 in a way that contrasts with the unbelief of Romans 1.
That is why it depends on faith, in order that the promise may rest on grace and be guaranteed to all his offspring—not only to the adherent of the law but also to the one who shares the faith of Abraham, who is the father of us all, as it is written, “I have made you the father of many nations”—in the presence of the God in whom he believed, who gives life to the dead and calls into existence the things that do not exist. In hope he believed against hope, that he should become the father of many nations, as he had been told, “So shall your offspring be.” He did not weaken in faith when he considered his own body, which was as good as dead (since he was about a hundred years old), or when he considered the barrenness of Sarah’s womb. No unbelief made him waver concerning the promise of God, but he grew strong in his faith as he gave glory to God, fully convinced that God was able to do what he had promised. That is why his faith was “counted to him as righteousness” (Romans 4.16-22).
Unbelievers turn from the true God to creation for their life and blessedness. Abraham saw nothing in creation that could help him but trusted in the living God for new life. Unbelievers give glory to created idols; Abraham gave glory to the true God. This is all described as Abraham’s faith. God counted that faith as his righteousness, a righteousness delivered to him in the death and resurrection of Jesus.
So justification is a verdict and, as a verdict, a deliverance from the curse of unbelief. While the verdict was declared in the resurrection of Jesus, and will be declared at the final judgment, God also declares it when, by his mercy alone, he turns sinners away from unbelief and grants them true faith in Himself through His Son.