While I do not think the objectivity of moral values makes sense in an atheistic or purely naturalistic world, many atheists and naturalists affirm the objectivity of moral values anyway (for which I am happy). When you press them to explain what makes it wrong to steal, rape, or murder, however, they will often respond that such things are morally wrong because they cause unnecessary suffering. This is unhelpful. The question seeks to know the ontological grounding for the moral values that exist in the world. Rather than provide that grounding, the atheist appeals to another moral value (any X that causes unnecessary suffering is wrong). But you can’t explain what makes moral values “moral” by citing another moral value. The moral value that it is wrong to cause harm unnecessarily needs to be grounded ontologically just as much as the moral value that it is wrong to steal or right to tell the truth needs to be grounded ontologically. Since it can still be asked what makes it wrong to cause unnecessary harm, the ontological grounding for morality must go deeper.
When you press the atheist to explain what makes it wrong to inflict unnecessary suffering on others, they will often evade the question by offering a counter-question of their own: “Well, don’t you think it’s wrong?” Apart from the fact that they have ducked the question by shifting the burden of proof on you, they have also changed the topic from moral ontology to moral epistemology. The question is not whether we can apprehend a realm of moral values (epistemology), but how to make sense of the existence of moral values (ontology) in the first place. To make this point clear, and to get the conversation back on track (and properly place the burden of proof back on the atheist/naturalist), perhaps you could respond, “Yes, I do think it’s wrong to inflict unnecessary suffering on others. This moral truth is obvious to all. What is not obvious is why it is obviously wrong to inflict unnecessary suffering on others if atheism/naturalism is true. It may only seem obvious to us because of our evolutionary past or sociological conditioning. The question you need to answer is not whether we can know what is moral, but what makes those moral values ‘moral’ in the first place.”