If I was an atheist, then I would be a nihilist.
A few years ago I bought and read the book Is Goodness Without God Enough? A Debate on Faith, Secularism, and Ethics. It's a series of essays generated by the debate, in 2001, between theist William Lane Craig and atheist Paul Kurtz.
I see Kurtz as missing Craig's two points, which are:
1) If theism is true, then we have a solid basis for morality.
2) If theism is false, then we do not have a solid basis for morality.
If 2 is true, then atheism does not lead to humanism (as Kurtz thinks), but to nihilism. Craig says: "If theism is false, you've got to ask yourself, Why wouldn't nihilism be true? What proof do you have that nihilism is not the correct remaining alternative?" (44)
If atheism is true it follows that humans are "just animals, and animals have no moral obligations to one another." (32) Craig quotes ethicist Richard Taylor, who invites us to imagine humans who live in a state of nature without any customs or laws, and one of them kills another and takes his property. Taylor writes:
"Such actions, though injurious to their victims, are no more unjust or immoral than they would be if done by one animal to another. A hawk that seizes a fish from the sea kills it, but does not murder it; and another hawk that seizes the fish from the talons of the first takes it, but does not steal it - for none of these things is forbidden. And exactly the same considerations apply to the people we are imagining." (32)
If atheism is true, it's hard for me to see that nihilism is not true.
This is not an argument for either theism or atheism. It is to present a hypothetical situation, with logical entailments. It is to argue against the rationality of moral obligation under atheistic humanism.
Finally it is not, as Craig so often tells us, to say that an atheist cannot act morally. Craig's point is about moral ontology, not moral behavior or moral epistemology. With these things in mind we can make sense of a statement like this:
Craig: "Thus, if atheism is true, it becomes impossible to condemn war, oppression, or crime as evil. Nor can one praise brotherhood, equality, or love as good. It doesn't matter what you do - for there is no right and wrong; good and evil does not exist." (33)
(See my How One Atheist Lives Without Morality)