Ambrosiaster wrote on Romans 1:17 the following,
"Paul says this because the righteousness of God is revealed in the one who believes, whether Jew or Greek. He calls it the righteousness of God because God freely justifies the ungodly by faith, without the works of the law, just as he says elsewhere: That I may be found in him, not having a righteousness of my own, based on law, but that which is through faith in Christ, the righteousness from God that depends on faith. He says that this same righteousness is revealed in the gospel when God grants faith to man, through which he may be justified...The truth and righteousness of God are revealed in this, when a man believes and confesses. The righteousness is of God because what he promised, he gave. Therefore, whoever believes that he has received what God promised through his prophets proves that God is just and is a witness of his righteousness...Through faith for faith. What does this mean, except that the faith of God is in him because he promised, and the faith of man is in him because he believes the one who promises, so that through faith of the God who promises the righteousness of God might be revealed in the faith of the man who believes? For to the believer God appears to be just, but to the unbeliever he appears to be unjust. Anyone who does not believe that God has given what he promised denies that God is truthful. This is said against the Jews, who deny that Christ is the one whom God promised...As it is written: "he who through faith is righteous shall live." Paul now moves over to the example of the prophet Habakkuk in order to declare that in the past it was revealed that a just man lives by faith and not by the law, i.e., that a man is not justified before God by the law but by faith." (Bray and Oden, Ancient Christian Commentary On Scripture, New Testament VI, Romans, pg. 31).
