When I was young, I heard a fair amount of argumentation that “atheists can be just as moral as Christians,” and in a very theoretical sense—and neglecting the duty of piety to God which an atheist must necessarily transgress—there is a sense in which this is true. As time has proven, though, it’s an irrelevant sense.
There are two possible meanings to “atheists can be just as moral”, though they end up in the same place. The first meaning is that an atheist can, through practice and effort, form virtuous habits according to whatever theory of morality he holds, and continue in these virtuous habits all the days of his life. This is true, especially for people who become atheists as adults, since adults tend to be fixed in their habits relative to children. Habits formed in youth are, certainly, possible to keep even if the reason for them has been rejected. It is not easy for such an atheist, since the continuation of his habits will have only the support of his prejudices and not of his intellect, but it is certainly doable. At least so long as he doesn’t face temptation. Upper middle class bachelors, with cheap hobbies, few wants, and no family obligations, will in fact tend to be harmless until they return to dust.
This misses is that there is more to morality than simply being harmless, or even than simply being “a productive member of society”. These are fine things, but human beings were made for greatness, not for being somewhat more convenient to their neighbors than absolutely nothing would be. It doesn’t really matter, anyway, because of the second possible meaning.
The second possible meaning of “atheists can be just as moral” is that they can be just as moral, according to their own moral standards. If this sounds good to you, consider that Genghis Khan thought that pillaging, raping, and murdering were heroic actions and that he was a really great guy for doing them. Being moral according to your own moral standards—assuming you’ve done your best to ensure your moral standards are correct—may possibly be sufficient to make you saveable by the salvific power of Christ on the cross atoning for the sins of the world, but it doesn’t mean that you aren’t being immoral according to the correct moral standards.
To say that an atheist can follow his own moral standards is, in fact, to say that he is liable to be immoral precisely to the degree in which his standards differ from real morality. It is no answer to say that he will approve of himself while he fornicates, adulterates marriages, and murders. It is no answer to say that he will only murder the people he believes it’s perfectly fine to murder, or that he will only murder people he declares aren’t human beings before he murders them. Approving of his evil actions doesn’t make him less wrong, it makes him more wrong.
When the famous enlightenment philosopher said, “there is no God, but don’t tell that to the servants or they will steal the silver,” it is no answer to say, “perhaps, but they will not think that it’s wrong to steal the silver while they steal it.”
This does bring up something of a side-note, which is sometimes talked about in this context: people who believe that God sees everything and punishes wrongdoing in the afterlife are more likely to think that they cannot get away with a moral transgression than are people who think that no one will catch them now or later. There is something to this, since the feeling that someone will know what you do is a support to doing the right thing. This is not the entirety of morality, however. People can habituate themselves to virtue such that they do not need this support in order to do the right thing, i.e. so that they can do the right thing even if they don’t believe anyone will know, merely because it is the right thing to do. Which brings us back to this second point: that does no good if the person’s theory of right and wrong is wrong.
If someone’s theory of right and wrong is wrong, he will not just do wrong (that he approves of) when no one is looking, he will also do wrong when everyone is looking.
Many atheists of the late 1800s and early 1900s took great offense at the idea that an atheist could not recognize moral standards. All people know right from wrong, they indignantly said. That modern atheists are busy villifying those older atheists for not living up to the different moral standards of modern atheists tells you what you need to know about this silly claim.
In the end, it doesn’t matter whether atheists can, through effort and practice, create virtuous habits and stick to them even if no one is looking, until the day they die. History tells us, if common sense didn’t already, that they will alter their moral standards before then. Once that happens, it’s only a matter of time and temptation before they form new habits accordingly.