Here Paul imagines someone continuing to reject the Christian gospel. That adversary argues, because his sin shows the righteousness of God, then God should not declare that he is a sinner on judgment day if, for example, he tells lies. (See: [[rc://en/ta/man/translate/figs-rquestion]])
Here Paul raises a question of his own, to show how ridiculous the argument of his imaginary adversary is. AT: "I might as well be saying ... come!" (See: [[rc://en/ta/man/translate/figs-rquestion]])