But we not only think it is wrong, we think everyone should agree that it is wrong. This is not to say that something is wrong just because everyone agrees it is wrong. There is a logical possibility that we are mistaken and it is just our cultural conditioning that tells us these things are wrong. If so, it seems this line of reasoning is self-refuting. It fails its own test.
Most people know that such atrocities as what the Holocaust are genuinely, objectively wrong.
Christians believe that if objective principles of right and wrong exist, there must be a foundation for them. And the foundation that makes the most sense is the character of a perfect and holy God. For more information on why God is the most logical foundation for morality, see Michael Horner's web site.