Paul said that having the Word of God, which instructs about what constitutes sin and how to receive forgiveness, made being Jewish an advantage. Yet, if many of the Jews did not believe, that didn't mean that God's plan of salvation didn't work or that God is unrighteous. Paul continued his thought by taking an argument to the extreme.
Romans 3:5-8 (KJV)
But if our unrighteousness commend the righteousness of God, what shall we say? Is God unrighteous who taketh vengeance? (I speak as a man)
God forbid: for then how shall God judge the world?
For if the truth of God hath more abounded through my lie unto his glory; why yet am I also judged as a sinner?
And not rather, (as we be slanderously reported, and as some affirm that we say,) Let us do evil, that good may come? whose damnation is just.
If mankind's sin contrasted so completely with God's righteousness, making God's goodness seem even more holy, lofty, and perfect in comparison, wasn't God unrighteous if He punished that sin (verse 5)? Paul was making an argument that conveyed the idea that God received a kind of benefit from sin's existence. He was "speaking as a man," meaning that this argument was of faulty construction. It was the type of argument or reasoning that someone might have, but it was not the truth.
Again, Paul objected vehemently with, "God forbid!" If God truly were unrighteous, then He would not be qualified to judge the world (verse 6).
Paul repeated his argument in verse 7 using the specific sin of lying as an example.
Then, in verse 8 the outcome of this flawed reasoning was taken to the conclusion that mankind should go ahead and do evil so that good would come! Paul added that there were those who had slanderously reported that Paul had actually taught this. However, the damnation of anyone teaching this doctrine was just.
Paul has been building a case for mankind's guilt before God concerning sin. Both Jews and Gentiles are guilty. Questions have arisen regarding the righteousness of God in His judgment, yet in all things God remains just. Stay tuned!