Are Gentiles who never had the law still guilty of sin?
Yes. Scripture teaches that Gentiles who did not have the Mosaic law still show the work of the law written in their hearts, their conscience bearing witness. Even without the Ten Commandments, they possess a moral system, so they are guilty of sin just as much as anyone else.
Les explains that Gentiles who have not the law of Moses do by nature the things contained in the law, showing the work of the law written in their hearts, with their conscience bearing witness. Though they did not have the Ten Commandments—such as thou shalt not steal, kill, or commit adultery—they still had a moral system based on conscience. Les illustrates this with missionary friends from the Amazon jungles of South America, where uncivilized Indians who hunt with blowguns and spears and think nothing of murdering people nevertheless have a tribal moral code that would put America to shame. Their source is conscience. Therefore, Scripture says that even though they did not have the law, they were guilty of sin just as much as anyone else.