Atheism is like chicken. It takes on the flavors of whatever culture or ideology surrounds it. There is no distinct set of atheist ethics because it is only a belief in the absence of God. Whatever system of morals or ethics an atheist subscribes to does not come from atheism, but something else. Most atheists in the U.S. live in a Christian culture, colored by a distinctively American individualism. So most American atheists, whether they know it or not, accept the broad strokes of Christian morality.
Moral relativism has become a popular theory, because it is a dangerously easy way to reconcile different opinions. There is no reason to argue about what is actually true if there is no truth. But even the most committed moral relativist becomes a moral absolutist when his house gets broken into, when he is cheated on, when he is insulted. Moral relativism is unstable and unsustainable. There is good, evil, and yes, shades of gray in between. The standards of right and wrong must be set in some way, transcending our ability to invent them.
Standards of right and wrong have been intimately connected to the divine for quite some time, but this was only solidified with a monotheistic system. Pagan Greece and Rome had standards of right and wrong, people were capable of both, but so were the gods. In the great monotheistic religions, the standard of goodness is set by one supreme deity who is goodness itself. Humans know the basic standards of right and wrong, there is a level of societal teaching necessary, but the basics are pretty universal. Easy enough.
We also know entire cultures often accept or promote certain kinds of evil. Our relativist sensibilities have made us uncomfortable with calling out any cultural practice as evil, but ancient Rome and Greece subjected infants to death by exposure to the elements, Aztecs sacrificed humans, even in the 20th century lynched people, and today we are comfortable with plenty that is morally questionable. What this says is not that morality is relative between cultures, only that cultures can and do pervert the truth. Religions fundamentally change cultures. We are able to express horror at the idea of exposing infants because Christianity challenged that widespread cultural practice.
Christianity forms the bulk of our ideas of right and wrong. Our belief in the fundamental equality of people stems from the Christian belief in being made in the image of God. Our indignation at the plight of the poor comes from a religion that believes that the poor are especially blessed. Our belief that those in relationships have a duty to be faithful to one another stems from the Biblical prohibition on adultery. The Greeks and Romans prohibited adultery as well, but women were always the guilty party. Judaism and later Christianity held both sexes were capable of infidelity.
The question is not atheists cannot be moral; they can and usually are. Most everyone of any creed is a decent person, seeking to uphold some standard. The question is one of origins. The atheist accepts a basically Christian morality because it still informs and shapes the culture. A morality that is purely atheist in origin looks almost alien, even to an atheist. Nietzsche rejected the Christian idea of “blessed are the meek,” and replaced it with “blessed are the strong.” He created a purely atheistic morality based on empowering one’s self. Almost nobody, I would guess not even Nietzsche himself, accepted this world view entirely and sincerely. Another, opposite atheistic system of morality, Marxism, uses Christian theology as its model. In Christianity, the kingdom of heaven is at hand. In Marxism, the revolution is at hand. What these atheistic conceptions of morality have in common is their assumption of the perfectibility of human beings. By man’s effort, we can create a brave new world. Because we are perfectible by own efforts, we can move the goalpost and bend the rules as we wish – because we created them. Religious systems, and Christianity in particular, recognize man is fallen, and something external is necessary for morality. No one has ever created a moral system from scratch because it will always be based, however imperfectly, on the natural law. It will either coincide on accident, or be flatly wrong. Anyone who is brought up in the Christian culture will accept Christian morals either because they have made a choice to accept them, or because they already believe them to be true.
The shell of a Christian world view still guides most people’s consciences. We believe killing is wrong, we believe in some standards for sexuality. Lying, cheating, hatred, are all wrong. But as more reject religion, a post religious morality is slowly taking shape, and has been for at least the last hundred years. The nuclear bomb is a post religious weapon. It has a moral implication, one based on a cold utilitarianism. Other evils, such as euthanasia, are based on sentimentality, a feel-good hollow compassion. It is a hatred for life, cleverly disguised as a hatred of suffering. As the Christian moral lineage becomes gradually replaced with saccharine sentimentality or cold utilitarianism, our descendants will likely occupy a moral space we will find unrecognizable.