Rom 3:5 But if our unrighteousness demonstrates the righteousness of God, what shall we say? Is God unjust who inflicts wrath? (I speak as a man.)
Rom 3:6 Certainly not! For then how will God judge the world?
Rom 3:7 For if the truth of God has increased through my lie to His glory, why am I also still judged as a sinner?
Rom 3:8 And why not say, “Let us do evil that good may come”?—as we are slanderously reported and as some affirm that we say. Their condemnation is just.

In Romans 3:8-9, Paul is addressing the Jewish prevailing thought that although the Jews are unrighteous, they will be saved due to their status as the chosen people. There was a claim among the Jews that Jewish unrighteousness was being used for God’s glory. The Jews being saved, although unrighteous, would contrast and amply God’s own righteousness, acting as a foil. The claim was that God would be unjust to exact vengeance on those He was using to amplifying His own righteousness. The entire verse 5 is Paul recounting this popular claim.

Paul next counters this claim. Paul offers several evidences. Paul’s first evidence is that if this was true, then to remain consistent, God would have to judge the Gentiles in the same matter. The Jewish belief was that the Gentiles would not be judged under the same standard, and the unrighteousness of the Gentiles would receive harsh judgement of God. Paul appeals to God’s consistency. Paul says God is not double minded, and God would be unjust to judge two separate groups on different standards.

In verse 7, Paul postures a hypothetical asking if what he is saying is morally right (as opposed to true). Paul says that even if he is lying, his lie is better than the Jewish portrayal. Paul appeals to moral consistency in God. Paul links God’s consistency to God’s glory. Paul is contrasting his view of God’s consistency with the Jewish claim of double standards.

This last verse has an odd translation in the KJV and NKJV. In Reading Through Romans, author C.R. Hume translates it as such:

Surely we are not to do evil so that good may come [of it] (as we are falsely accused of saying, as some people allege, and their condemnation is just)?

Paul takes the opportunity to address a false rumor about himself that parallels the Jewish claim. Rumors were circulating that Paul himself was teaching that Christians who sin then amplify God’s grace. Paul says this is not true, and, by extension, this Jewish claim was also not true. Paul states that condemnation of this view is “just”.