Atheists are often accused of having no solid foundation on which to base their moral or ethical codes and are therefore assumed to be, by necessity, moral relativists. I disagree. I'm not going to argue that most atheists are actually moral absolutists because I honestly have no idea and I doubt a poll has ever been done on the subject. My argument is rather that morality does appear to be relative (that is, culturally and time specific) to an extent but that that doesn't mean moral relativism ought to be espoused. In fact, we have the ability to rationally motivate the choice of a given ethical paradigm over a variety of others. Furthermore I hold that religious tradition is a terribly unstable foundation for an ethical code and that our knowledge of facts of nature can provide a far better one.
Religions are no sufficient standard. I have always thought that if you embrace an all-encompassing metaphysical "reality," you can only ever be as good as the god(s) or, more generally, the entities that supposedly inhabit it. And vice versa. Considering how most deities ever conjured into existence - figuratively - by human imagination have all shared the one feature of displaying the basest of human drives and behaviours, it is not surprising how most of our history has not been particularly praiseworthy in moral terms. That has set an interesting phenomenon in motion, by which divine laws would last only as long as the god(s) that issued them (and the particular social customs of the specific population), only to be replaced over time by updated divine whims and new customs altogether. This absurd situation has made the apologists' work a living nightmare, as they attempt to reconcile unreconcilable moral tenets, all the while maintaining that their deity's word is immutable and that moral relativism is a devilry. Take the Mosaic covenant as an example, most of which is dispensed with in the New Testament on the grounds of changed cultural and social circumstances - a textbook definition of divine moral relativism. So, you see, your average modern Christian apologist, meddling and interfering with modern policy-making, is in the uncomfortable position of having to decry a "certain dominant moral relativism" while actually acknowledging that their deity once sanctioned practices that would consensually be considered as immoral and unethical by most modern standards, simply because allegedly required at one time to keep god's chosen people at bay. All this obviously doesn't even begin to address the issue of which particular religious code among the hundreds or thousands in existence should actually be considered the true one, and the primacy of one (or three) of them over the rest appears entirely arbitrary. There you have it. No religious creed can coherently form the basis of a functional social contract capable of supporting the evolution of our civilisation. Religious creeds can only communicate to us - or impose on us - the demands of this or that deity but they will always fail to provide a rational justification for the dos and don'ts necessary for social living. Not to mention that they're too unreliable, just like said deities, and prone to the very social relativism they wish to condemn.
Now, here's the problem. Once god(s) have lost their moral high ground, such a change leaves a noticeable hole in our justification for morality, at least according to some. That is, the social contracts used to manage our society become meaningless to some people - or so they like to claim - unless we can find a definition for the "good" that social contracts aspire to. Apparently something cannot be "good" if not by divine decree. Fortunately for us, that's ridiculous. We have pretty good ways to support a definition of "good" and "right" without resorting to the metaphysical. We could easily define as "good" all those actions and behaviours that can help us maximise well-being for the highest number of people. Some might argue that the very definition of "well-being" needs to be stated and agreed upon, but that is just being captious. Artists and philosophers have been writing about happiness for millennia, and most of us experience happiness to varying degrees frequently enough to create a consensus as to what constitutes "well-being." It then becomes obvious that some of the most important ethical tenets forcibly upheld by religions under pain of eternal torture and because "god says so," become simply self-evident. A society in which murder, theft and lying are routinely practised by the majority of people would simply be a short-lived one. A society in which one's belongings and one's well-being are constantly at risk is a society with no cohesion and social bonding, and it's bound to break down. A society in which the normal exchange of accurate and reliable information is made impossible by consistently lying to each other all the time would meet the same fate. Additionally, a successful society is one in which poverty is minimised as much as possible; loneliness is reduced in favour of social cohesion, a sense of belonging and purpose. When the focus is shifted from whatever personal relationship with the metaphysical to the individual well-being as a function of the collective well-being, we finally have a rationally-motivated notion of "good" to pursue. No metaphysical religious belief can offer that.