Atheists believe in nothing
Theists often respond to assertions of atheism with the question, "So you believe in nothing?" Such a question not only patently mischaracterizes the nature of atheism, but it betrays one of the inherent problems in Christian doctrine: the potential for development of a type of "Christian nihilism," in which adherents see all meaning and value as derived only from God.
Guidelines not needed
This question is similar to accusations that atheism is immoral since it lacks guidelines dictated from a "higher power." Therefore, asking this question essentially implies that humans are incapable of independent and complex thought, a concept debunked in numerous examples worldwide. Consider the fact that many Buddhists, despite having complex systems of dogma and spiritual beliefs, are atheists by definition because they reject the concept of god in any monotheistic sense. While some schools of Buddhism differ from this interpretation, there are many Buddhists practicing what they view as a rich and complex religion, while asserting that their beliefs and dogma come not from a divine authority.
How, then, is a secular atheist fundamentally different from a non-theistic Buddhist who doesn't believe in God, yet clearly doesn't believe in nothing? Countless schools of philosophy serve as a basis for development of secular belief. Consider (secular) existentialism, which revolves around the concepts of self-discovery and finding meaning and value relevant to one's own life and worldview. Many philosophers and self-described existentialists have written works detailing quite complex systems of epistemology (in a sense, "What can we know?"), morality, and more, without ever involving God.
Whether or not atheists describe themselves as existentialist, the framework of existentialism shows how one may find belief, meaning, and value in a purely secular way, outside the bounds of any defined religion.
Theistic Response: Atheism anticipates Relativism
A theist could respond by saying that any attempt at finding belief, meaning, and value apart from the existence of God will inevitably result in a vicious relativism, where meaning and value are contingent and are not necessary or absolute. Although this line of reasoning does not necessitate belief or direct knowledge of a transcendent Christian God, it does raise the issue of the lack of objective, absolute determining criteria for meaning and value. Even within the framework of existentialism, one must either consent to the existence of an absolute moral guideline (i.e., God) even if one does not adopt a particular religion, or one must admit there might not be any absolute, objective and necessary values and meanings. (The first side or the theists in question would have to prove to begin with their claim that there really exists such a thing as absolute morality - "True purpose" for every being; in case they succeed in this part, they still have to properly demonstrate what this essential meaning factually is, not just resort to personal opinions of what one wants it to be.)
Christianity as contingent
Incidentally, Christianity is a contingent belief system and Christian morality is a contingent moral system because different Christians, different Christian sects, different Christian cultures, different Christian nations, Christians at different times in history, etc., have interpreted matters of faith and morality differently.
Another way of putting this is that Christian beliefs and Christian morality are contingent on individual interpretations or on the interpretations of any given sect, culture, nation, time in history, etc.