The moment you say that one set of moral ideas can be better than another, you are, in fact, measuring them both by a standard, saying that one of them conforms to that standard more nearly than the other. But the standard that measures two things is something different from either. You are, in fact, comparing them both with some Real Morality, admitting that there is such a thing as a real Right, independent of what people think, and that some people's ideas get nearer to that real Right than others.
Or put it this way. If your moral ideas can be truer, and those of the Nazis less true, there must be something-some Real Morality--for them to be true about.
If the Rule of Decent Behaviour meant simply 'whatever each nation happens to approve,' there would be no sense in saying that any one nation had ever been more correct in its approval than any other; no sense in saying that the world could ever grow morally better or morally worse.
CS Lewis, Mere Christianity