Where did people who never had the written law get their knowledge of right and wrong?
God placed His law within them—in their hearts. Paul explains in Romans that people who did not have the written law still possess an inner moral code; their conscience either accuses or excuses them, so their knowledge of right and wrong comes from God's law written on the heart.
Les explains that even uncivilized tribes have a moral code, one that many times would make America look pretty sick within their own tribe. The question is where they get it. Paul states in Romans that for those who did not have the written law, God put the law in their hearts—the conscience, meanwhile, accusing or else excusing them. Even the uncivilized peoples of the world, in order to maintain their little tribe, have to have some sort of law patterned after what God gave to Moses.
What shall we say then? Is the law sin? God forbid. Nay, I had not known sin, but by the law: for I had not known lust, except the law had said, Thou shalt not covet.