Paul is doing everything he can to unpack for his readers, brand new Christians in the depraved culture of Rome, how important it is to believe in Jesus as the source for their right relationship with the God of gods. But in addition to that he presses into the worldview of those who aren't convinced that they need to be concerned with their sin and eternal standing with the Divine by creating a scenario where he builds the case for the necessity for believing in Jesus even if sin wasn't a real issue.   To be clear this episode should be understood in the context of the entire book of Romans and is not meant to be presented as a stand-alone presentation of the gospel or the reason Paul is asserting for putting one's faith in Jesus Christ.