There are two main moral camps under which you can classify atheists. Of course there are various shades and degrees within the respective camps, but there is a continuum upon which they all fall. On one end there is 'humanism' which extols various virtues and values such as justice, truth, altruism, love etc., as well as recognizing their opposites as evil. On the other end of the continuum is 'nihilism' which holds that there is no such thing as virtue or value, only arbitrary human preference. The vast majority of atheists in the world fall on the 'humanist' side of the continuum.
The humanist philosophy is internally contradictory and incoherent, while the philosophy of nihilism, which has exponentially fewer adherents, is actually fully coherent. I don't believe that either is true, but nihilism at least fully rejects the notion of values and morals as meaningful outright, while humanism attempts to retain these things, while simultaneously denying their foundation. Without an external authority to define them, values must be arbitrary personal preference, as the nihilist rightly claims. If values are arbitrary preference, then secularists must not recognize any act or belief as good, nor condemn any act or belief as bad. Yet this is exactly what the humanist does.
The scientific-minded modernist will undoubtedly interject at this juncture that morals have a natural, evolutionary, socio-biological origin, and can be understood in such a framework. However, even if you precisely understood the evolutionary origin of morality, that still doesn't get you to a reason to be moral. All it gets you is an explanation of the history of morality. But morality is ultimately the question of how we are to behave going forward, not an explanation of how we have behaved up to this point in time. So this objection is a canard; a socio-biological mutual-benefit understanding of morality does not yield any basis for values going forward, only a means for understanding the facts of how we have behaved in the past. The 'is' does not beget an 'ought'.
Is there any reason that I should be obliged to act in accordance with these innate socio-biological guidelines, rather than defy them? The scientific modernist would now say the reason to do so is that our socio-biological morality has developed in order to preserve order and ensure the survival and propagation of the species. But if I, as an individual, do not highly value any of these things (i.e. survival, the species etc.), then what basis do you have to tell me that my decision to act directly against these socio-biological moral structures is "wrong"? At this point the honest scientific modernist says "Well, you're right, there is no basis to do so, but as a society we can still enforce and uphold these values if we choose to." Which is a tacit submission of defeat; there is in fact no basis for values or morals outside of an external, divine order.
So the humanist necessarily concedes that values are arbitrary preferences, while continuing to assert forcefully that the environment must be protected, that homosexuals must be granted the right to marry, that racism is deplorable etc. Mr. Humanist, don't you actually mean: "racism is deplorable... or not"?
In one sense this refusal to jettison value is to the humanist's credit. In refusing to do so they are responding to a deeper, truer human impulse; the divinely-authored conscience. Their insistence on extolling certain values and virtues is only inconsistent with strict naturalist maxims, not with reality. It's perfectly consistent with reality. But their position is internally inconsistent and incoherent as it can't be reconciled in a world without objective value, which is the world which they claim to occupy. You can't get to "objective" anything without an ultimate authority, therefore they hold two beliefs (one implicit and one explicit) that are necessarily mutually exclusive.
The humanist might respond by saying that he realizes his values are merely subjective ways of thinking about things, but still chooses to hold them, which is of course fine. But if that were true then why would he feel the need to condemn any other way of thinking about something? Why condemn any other act? Why condemn Hitler and laud Gandhi, rather than state that which must be true: "here are two men who had different visions of the world", and nothing else? It's important to note here that in a socio-biological sense, even deviants and sociopaths are themselves hardwired socio-biologically. They simply represent an aberration to particular social norms. An aberration which itself could theoretically serve some socio-biological function, population control for instance. So if your basis for morality is based on evolutionary social mechanisms, then the aberrations can not be said to be any better or worse than the social norms themselves; they simply are what they are.
I think it's important to ask at this point "what is my goal in writing this"? Certainly it isn't to convince humanists to embrace nihilism, but rather to attempt to convey that the fact that you do hold particular values and believe in virtue is itself evidence of a divine creator. Socio-biological morality is at best an explanation of historical moral norms that have happened to arise, not a basis upon which you are to make moral judgments in the future, or decisions about how you should behave. As an atheist if you find some acts so repugnant they must be condemned, or other acts so virtuous they must be praised, then good, you ought to. Just realize that these feelings of yours can only have one of two possible origins: (A) purely naturalistic socio-biological norms, in which case the repugnant can't be said to be worse than the praiseworthy in any real sense other than your singular mind happens to feel that way, but rather must be recognized as an acceptable, natural deviation from the norm or (B) a divinely-authored conscience which allows all men to be able to distinguish good from evil, which in turn necessitates accountability for our words, actions and beliefs.