‘Isn’t Moral Law just a social convention?

People who ask that question are usually taking it for granted that if we have learned a thing from parents and teachers, then that thing must be merely a human invention. 

Multiplication Table: Moral laws are like multiplication tables though we learned it in school yet it is not a mere human invention, if it was they might have made different if they had liked?

Walking on the road: If morality was just a human convention like we keep to the left while walking on the road. We can change it and make a new rule to walk on the right. But not so with the multiplication table we can't change it the way we like.

Morality vs Morality: If these were just mere human conventions than we can say that morality of one nation is better than another or morality of one culture is better than another. 

Because if no set of moral ideas were truer or better than any other, there would be no sense in preferring civilised morality to savage morality, or Christian morality to Nazi morality. But of course, we all do believe that some moralities are better than others.

Measuring Standard: 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, 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.

New York: The reason why your idea of New York can be truer or less true than mine is that New York is a real place, existing quite apart from what either of us thinks. If when each of us said ‘New York’ each means merely ‘The town I am imagining in my own head,’ how could one of us have truer ideas than the other?

I conclude then, that though the differences between people’s ideas of Decent Behaviour often make you suspect that there is no real natural Law of Behaviour at all, yet the things we are bound to think about these differences really prove just the opposite. 

Source: Mere Christianity by C.S Lewis