Here is another passage from Mere Christianity, the classic by C. S. Lewis that spells out what all Christians believe.
"If no set of moral ideas were truer or better than any other, there would be no sense in preferring civilized morality to savage morality, or Christian morality to Nazi morality. In fact, of course, we all do believe that some moralities are better than others. We do believe that some of the people who tried to change the moral ideas of their own age were what we would call Reformers or Pioneers--people who understood morality better than their neighbors did. Very well then. 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 there is such a thing as 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."