Everyone has a worldview. Every worldview has a set of core beliefs through which persons view the world; viz., through which they construe reality and experience. For example:
- In the worldview of Chinese primitive religion one's ancestors dwell in an afterlife and still need to be provided for. Hence, this belief: One's ancestors' well-being depend on sacrifices offered to them in this life.
- When I was in Kenya I learned that, in Sub-Saharan Africa, divinization rites are an essential part of daily life. I was told that in Kenya, e.g., "diviners are everywhere." So, one belief in the worldview of African primitive religion is: casting pieces of a kola nut can tell a person what to do to make it safely through the day.
- I embrace the worldview of Christian theism. One of my beliefs is: When I die I shall continue to exist, in the presence of God, in an afterlife (on the basis of Christ's historical resurrection).
Here, e.g., is a belief, an atheist would [and should] have: morality does not exist. An atheist's reasoning would be this:
- If there is no God (as moral Commander), then there is no morality (no objective moral values).
- God does not exist.
- Therefore, objective moral values do not exist (morality does not exist).
Am I claiming that an atheist cannot be moral? Not at all. In fact, I know some atheists who are more moral than some Christians. My point (as made by other atheists and theists) is that, on atheism, there is no longer any metaphyscial foundation for morality.
Important to understand here is that this reasoning is not about describing how moral beliefs come about. To describe how moral beliefs came about (moral epistemology) is irrelevant to this discussion. That is a matter of moral epistemology. The issue is, rather, a matter of moral ontology. It's not a matter of normative ethics; it's a metaethical issue.
As theistic philosopher William Lane Craig says, "The claim that moral values and duties are rooted in God is a Meta-Ethical claim about Moral Ontology, not about Moral Linguistics or Epistemology. It is fundamentally a claim about the objective status of moral properties, not a claim about the meaning of moral sentences or about the justification or knowledge of moral principles."
On atheism, objective moral values (i.e., Marks's "morality") do not exist. I find that eminently logical. An atheist who thinks otherwise is misguided and is, following Nietzsche, still too indebted to the village of Christian theism.
Note: Without morality, what does an atheist who has moral inclinations do? One example is Marks, who rejects utilitarianism and, following his Kantian inclinations, proposes something he calls "desirism." See "An Amoral Manifesto: Part I," and "An Amoral Manifesto: Part II."