Why does Paul repeat the salvation message over and over in Romans?
Paul repeats it because the salvation message is so weighty and important that God wants it grasped clearly: salvation cannot be by anything man can do, but is fully accomplished by Christ's work of the cross, and God remains just in justifying those who believe.
Les explains that Paul's repeated "therefore" and "wherefore" mean the reader should go back and remember what he just said. That produces constant repetition in Romans: the material is so heavy and so important that it is repeated over and over for about two or three chapters. The reason is that God wants us to get it straight—salvation can be by nothing man can do; it is all accomplished by His work of the cross. God remains just as the justifier of those who believe, not cutting corners or winking at man's sin, because He paid the total price. Paul also keeps bringing the two into play: by the offense of one, judgment and condemnation came upon all men, even so by the righteousness of one.
Therefore as by the offence of one judgment came upon all men to condemnation; even so by the righteousness of one the free gift came upon all men unto justification of life.