Romans 3:5-8 –  But if our unrighteousness serves to show the righteousness of God, what shall we say? That God is unrighteous to inflict wrath on us? (I speak in a human way.)  By no means! For then how could God judge the world?  But if through my lie God’s truth abounds to his glory, why am I still being condemned as a sinner?  And why not do evil that good may come?—as some people slanderously charge us with saying. Their condemnation is just.
If God’s grace triumphs over our sin then wouldn’t it make the grace of God appear more glorious if we sinned with abandon? This is the question that the Apostle Paul supposes someone will ask. Or perhaps someone has asked it.
This is not the only time that Paul will suppose someone asking if the Gospel means that our sin is a good thing. In 6:1 he will ask if we can sin to make grace abound. The answer is that the grace of God actually changes people into people of righteousness. We have been saved from sin. How can live in it any longer?
In chapter 9, while speaking about God’s right to love Jacob and hate Esau,His right to show mercy or withhold it, he asks that if this is true, then how can God hold anyone guilty? The answer there is that God can do whatever He likes.
Here, in Romans 3, Paul simply says that the thought that God could do something unrighteous is preposterous. He presupposes the absolute righteousness of God and says that to ask the question is to demonstrate how deserving of judgement such a thought is. It is to charge God with sin and that is unthinkable. God is the judge of all the earth and Paul reasons here that if He were not absolute in His righteousness He would be unable to judge at all.
The argumentation is beautiful. Paul is not afraid to presuppose certain truths about God. And the one thing that he will not abide is the thought that we can sit in judgement upon God. God is God and we are not. Whatever questions we have regarding the Gospel cannot impinge upon His righteousness. We are not more righteous than He is and to charge Him with wrong is to maintain that we know better then He does.
That is indeed very helpful. We must, in our conversations with others about the Gospel never allow the righteousness, the goodness, the sovereignty of God to be questioned. Question it, some will. But we will not go there. We reply that such thinking puts us above God and that is not possible. We must never assume that we have all the answers. When a text makes God look bad we assume a few things: 1- we are not understanding the text properly, 2 – we may be imposing our values upon the Almighty, 3 – we need to investigate things more deeply. It is not wrong to assume that God is God. We start with this.
God is God and what the Bible tells us about Him is true. It is a great comfort to the believing heart.