Here's another passage from the Christian classic by C. S. Lewis about what all Christians believe, Mere Christianity.
"Other people wrote to me saying, 'Isn't what you call the Moral Law just a social convention, something that is put into us by education? I think there is a misunderstanding here. The 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. But, of course, that is not so. We all learned the multiplication table at school. A child who grew up alone on a desert island would not know it. But surely it does not follow that the multiplication table is simply a human convention, something human beings have made up for themselves and might have made different if they liked? I fully agree that we learn the Rule of Decent Behavior from parents and teachers, and friends and books, as we learn everything else. But some of the things we learn are mere conventions which might have been different--we learn to keep to the left of the road, but it might just as well have been the rule to keep to the right--and others of them, like mathematics, are real truths. The question is to which class the Law of Human Nature belongs."