In many ways the letter to the Galatians is a mini-version of the letter to the Romans. Because of their similar themes, it's not surprising that Paul deals extensively with justification in writing to the Roman Christians.
Toward the end of Romans chapter three, after Paul has made an exhaustive case that all humanity is guilty before God, the apostle writes this (3:20), "For by works of the law no human being will be justified in his sight, since through the law comes knowledge of sin." From this we can conclude a couple of things. First, no one is justified (declared both not guilty and righteous) by works. Second, if anyone is to be justified, it will be on the basis of something other than works.
We are briefly left wondering what the basis of the justification will be.
Paul answers this question with the clear affirmation that justification comes through faith alone. In what is one of the most important paragraphs in all scripture, Paul writes (3:21-26), "But now the righteousness of God has been manifested apart from the law, although the Law and the Prophets bear witness to it - the righteousness of God through faith in Jesus Christ for all who believe. For there is no distinction: for all have sinned and fall short of the glory of God, and are justified by his grace as a gift, through the redemption that is in Christ Jesus, whom God put forward as a propitiation by his blood, to be received by faith. This was to show God's righteousness, because in his divine forbearance he had passed over former sins. It was to show his righteousness at the present time, so that he might be just and the justifier of the one who has faith in Jesus."
Paul tells us that we are justified by God's grace through faith in Jesus Christ, who is our redemption and propitiation.
As if to further make his case, Paul says (3:27-28), "Then what becomes of our boasting? It is excluded. By what kind of law? By a law of works? No, but by the law of faith. For we hold that one is justified by faith apart from works of the law."
In Romans chapter 4, Paul points back to two important figures from the Old Testament to show that even then justification came through faith apart from the law. The two people Paul selects are two of the most prominent in both Jewish history and in the ancestry of Christ: Abraham and David.
In Romans 4:1-8, Paul writes, "What then shall we say was gained by Abraham, our forefather according to the flesh? For if Abraham was justified by works, he has something to boast about, but not before God. For what does the Scripture say? 'Abraham believed God, and it was counted to him as righteousness.' Now to the one who works, his wages are not counted as a gift but as his due. And to the one who does not work but trusts him who justifies the ungodly, his faith is counted as righteousness, just as David also speaks of the blessing of the one to whom God counts righteousness apart from works: 'Blessed are those whose lawless deeds are forgiven, and whose sins are covered; blessed is the man against whom the Lord will not count his sin.'"
Paul goes on to inform us that both Jew and Gentile are declared just because of faith and not works. This is all a gift of God based on the righteousness of Jesus Christ.
Romans reads much like a lawyer making a logical case for the gospel. Step-by-step, Paul lays out God's plan before us. After seeing that all are guilty because they have all transgressed God's law, we read that no one can be justified because of the works of the law.
At first this seems like terrible news. If we can't be counted righteous by works of the law, then how can it possibly happen? This is when we read, beginning in 3:21, of the almost unbelievable truth that God declares us just because of a gift of His grace through our faith.
As in Galatians, Paul makes the clear contrast between faith and works. He leaves no room for a faith-plus-works scenario. His entire emphasis is upon justifying faith.
Both of these epistles cry out that justification comes by faith alone.