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 only derived from God.
Guidelines not needed
In a similar way that accusations that atheism is immoral, as it lacks guidelines dictated from a "higher power," this question essentially implies that humans are incapable of independent and complex thought, a concept heavily debunked in numerous examples worldwide. Begin by considering the fact that many Buddhists, despite having complex systems of dogma and spiritual beliefs, are atheists by definition, in that they reject the concept of god in any monotheist sense. While some schools of Buddhism differ from this interpretation, there are significant numbers of people practicing what otherwise fits the definition of a religion, while asserting that their beliefs and dogma come not from a divine authority.
How, then, is a secular atheist any different? Countless schools of philosophy serve as a basis for development of secular belief. Consider (secular) existentialism, which revolves around the concept of self-discovery and finding meaning and value in a way 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 atheists describe themselves as existentialist or not, the framework of existentialism shows how, outside of the bounds of any defined religion, one may find belief, meaning, and value in a purely secular way.
Theistic Response: Atheism anticipates Relativism
However the 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 an objective, absolute determining criteria for meaning and value. Even within the framework of existentialism, one must either consent to the existence of an absolute being, i.e. God, even if one does not adopt a particular religion or one must concede all absolute, objective and necessary values and meanings.
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 that is that Christian beliefs and christian morality are contingent on the individual interpretations, the interpretations of any given sect, of any given culture, of any given nation and of any given time in history etc.