What is the source or origin of these moral principles? If naturalism is true, shared moral values and duties must be the product either of evolutionary development (that is, they are hardwired into us biologically) or of cultural development (that is, they are instilled in us by society).
Notice that neither of these options allows moral values and duties to be something objective (true regardless of our opinions). On the evolutionary view, moral values and duties originated as features that helped our ancestors survive and reproduce (the same way the claws of a tiger help it survive or the colorful feathers of a peacock help it reproduce), but this has nothing to do with objective right and wrong. The statement "It's good to be compassionate" wouldn't be true. Instead, the true statement would be: "Compassion helped our ancestors pass on their genetic information." Our belief that compassion is objectively good would be a kind of illusion. Of course, one might think that whatever helps survival and reproduction must be good. But bedbugs reproduce through violent rape. The violent rape helps the species survive and reproduce, but this says nothing about whether violent rape is right or wrong.
On the cultural view, moral values and duties developed because they were needed by society. A society without rules is impossible, for the absence of rules would lead to chaos. But here again, moral values and duties are not objective. The statement "You should do unto others as you would have them do unto you" wouldn't be true. Instead, it would mean "Doing unto others as you would have them do unto you helps society." One may suggest that whatever helps society is good, but various societies have thrived on cruelty, oppression, and subjugation. Does this mean that cruelty, oppression, and subjugation are good?
Hence, if moral values and duties are going to be something beyond biology and culture, they need a transcendent source. This is the basis of the Moral Argument for God's Existence.