Why can God be just while justifying sinners?
God remains just in justifying sinners because He did not cut corners or wink at man's sin; He paid the total price for sin through His work of the cross. Salvation cannot be by anything man can do, and God is just as the justifier of those who believe.
Les teaches that Paul’s “therefore” calls the reader back to what was just said, and this heavy truth is repeated because God wants it straight: salvation can be by nothing man can do. It was all accomplished by His work of the cross. God is just in being the justifier of those who believe, not cutting corners and not winking at man’s sin, because He paid the total price for it. On that basis He can be just in justifying the unbeliever. Les also notes Paul’s constant contrast: by one offense condemnation came upon all, even so by the righteousness of One.