It is often argued that theism provides a basis for a coherent system of morality, whereas atheism offers no such guidance and indeed suggests that there can be no such thing as morality traditionally understood. It is just as often responded that atheists are moral people too and there is no evidence that conversion to atheism leads people into gross immorality or crime.
True enough, but that is not really the point. The issue is not, as I put it in my article Is it Possible to be Good Without God?, whether an atheist can be a good person, but whether goodness and evil are concepts sustainable in an atheistic milieu.
However, if atheists are just as moral, if not more so, than self-identified Christians, is this a distinction without a difference? Does it matter that the atheist's morality system may not be coherent so long as he acts morally?
Such thinking is shortsighted. Today's atheists have the benefit of 1500 years of Christian morality. This morality has affected all, not just strict Christian adherents. It has shaped an entire society's norms and expectations. The real question is not whether an atheist raised with morals framed by Christian influence will be moral, but what society will look like after generations of atheist triumph.
This issue was ably addressed by a recent article in Biblical Worldview Magazine, "Why Atheists Are Theocrats," by Gary DeMar. Here is how DeMar puts it:
If atheists get their way, they will be running the world in terms of some ultimate principle. At the moment, atheists have the benefit of a vibrant Christian worldview where they can borrow moral plugs like compassion and kindness to keep their hole-filled materialist boat afloat. Given time, future generations of atheists will logically throw off these moral precepts that at one time had been mined from "ancient literature." Consistency will lead these newly empowered atheists to conclude that "kindness" is a superstitious remnant of an ancient book-led religion that once proposed that immaterial entities exist. Science will show that there is no way to account for these religion-defined virtues given naturalistic assumptions.... When atheists no longer have Christianity to borrow from, from what bank will they draw their moral capital?