John C. Wright points to an aspect of atheism I observed myself when I was writing the book that was published as The Irrational Atheist:
There are many brands of atheism, but they all have some points in common. First, one common point is that none have a rational explanation of the objectivity of moral rules.
Not all cultures agree on what priority to place on various moral rules, but one thing that is so obvious about moral rules is that they are objective. When guilt pricks us, it does not say we betray a matter of taste or opinion; the feeling of guilt is the feeling of having offended a law. When injustice rankles, we do not accuse those who trespass against us of having breached a matter of taste or opinion; we refer to a standard we expect the other to know and acknowledge. We cannot help it.
In all human experience, everything is open to doubt but this. No man with a working conscience can escape the knowledge. It is the one thing we cannot not know. And yet atheists are at a loss to explain it.
I do not call atheists immoral, but I note they cannot give a rational reason to account for morality.
In any atheist worldview, moral laws are an invention of man and serve his contingent purposes, or an imposition of Darwinian survival mechanisms that serve the contingent purposes of the Selfish Gene. Such purposes as the preservation of life or the pursuit of happiness are subjective, hence not laws at all. Whether selected by nature or by man, if moral maxims are selected merely as a means to an arbitrary end, they are merely expedient conveniences.
In the end, nihilism is the only rational atheism. And since most atheists rightly shy away from nihilism and its concomitant despair, they tend to trap themselves into philosophical unreason. Which is, of course, is often more than a little ironic due to the oft-heard atheist claim to be a true devotee of reason.
They call conscience an illusion and morality an artificial construct. But if they are not correct (and there is considerable logic and evidence to conclude that they are not) then even the most staunch atheist must admit that he is the delusional one.