John Loftis, at his blog Debunking Christianity, lists ten objections, concepts, or topics that seem to be raised most often by atheists in debates with theists. Here's the seventh (my response to the earlier objections can be found by scrolling down the page):
7. The link between theism and morality has been conceptually (Euthyphro dilemma), empirically (evolutionary ethics), and culturally (morality existing without theism) discredited. Thus coupling God with the notion of Good is not only misleading, but trying to own a fundamental aspect of the human condition.
Each of the above claims is false. The Euthyphro dilemma has been repeatedly shown by numerous philosophers not to have the force that its critics believe it to have. See the Viewpoint posts of 4/27/15, 4/28/15, and 4/29/15 for a fuller treatment.
Evolution, if we are to take it to be a blind, impersonal force, may have instilled in us certain moral intuitions, but blind, impersonal forces cannot impose an obligation upon us to live one way rather than another. Moreover, evolution has produced a species (us) prone to cruelty as well as kindness, violence as well as gentleness, selfishness as well as selflessness. So on what basis do we decide one of each of these pairs is right and the other wrong?
As prominent evolutionary biologist Richard Dawkins wrote, "The universe we observe has precisely the properties we should expect if there is, at bottom, no design, no purpose, no evil and no good, nothing but blind pitiless indifference." Philosopher of biology Michael Ruse has written that morality is an illusion, fobbed off on us by evolution to get us to cooperate with each other, and philosopher Richard Rorty adds that for the secular man such as himself there's no good answer to the question, "Why not be cruel?" In other words, evolution gives us no reason whatsoever to be bound to whatever moral intuitions we might have.
Unless there is a God there can be no objective moral duties or values. There can only be ways of living based on one's personal preferences and feelings. How, after all, can there be a duty to do anything unless one is somehow held accountable for what one does? Nevertheless, almost everyone feels strongly that something like raping children or abusing the elderly is objectively wrong, but only the theist can say that it actually is wrong. The atheist has to admit that her sense that these things are wrong is simply a holdover from a blind process that fit us for life in the stone age and which can impose no obligation on us to heed it.
This is not to say that atheists are ipso facto going to behave badly. Atheists can be as kind, loving, and honest as the next person, but the point is that on atheism there's nothing but their own subjective preference that tells them they should be this way. If they adopted the opposite values they wouldn't be in any sense wrong, they'd just be different. This point is a major theme in my novel In the Absence of God.
Finally, it's not that belief in God is necessary for one to be morally good. One can believe in God and still not know what's right, and one can know what's right and still not do it. On the other hand, one can disbelieve in God and still do what is good. Rather, the point is that unless God exists there just isn't any duty to live by any particular values at all.
Unless God exists moral goodness is pretty much an empty concept.