Great question and thank you for asking it. Let’s look at the few verses surrounding what your asking about.
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.”
The beginning statement here is a question that Paul is asking in the form of someone else. He is responding to an idea some people have that if they are unrighteous, but their unrighteousness reveals God’s righteousness, then it’s really not a bad thing and God shouldn’t judge them. It’s a twisted doctrine that people were using to justify sin, and Paul condemns it. How could God judge the world if He was just going to write people off as forgiven for making Him look good by being sinful? God brings good out of our wickedness, and does bring glory to Himself through that, but it’s still our wickedness and we are still guilty of it before God.
Jesus said in John 14:15, “If you love me, you will keep my commandments.” We who are truly saved are those of us who have accepted Christ and followed His commandments. We do not get out of our sin by making the excuse of, “hey God I know I did a lot of bad stuff, but it made you look awesome though so…how about a free pass?” We are still guilty of our sin if we have not accepted Christ, and those who accept Christ will resist sin. Not perfectly of course, but they will not live in it, not for any reason or excuse.
The lie that Paul is referring to here is a continuation of his reference to what other people might be saying. He’s basically saying that there are people who might say, “but if my lie causes God’s truth to abound then it’s a good thing and He shouldn’t hold me accountable.” He doesn’t even bother to answer such a slanderous statement directly. Rather, he simply says that people who believe such things are condemned, and justly so for such slanderous beliefs.
I hope that answered your question. If you have any other questions please ask them. May God grant you wisdom and understanding as you study His word. God bless you and thank you.