Atheism/Agnosticism a Belief System?
Everyone has a religion. Everyone's life is guided by some principle or set of principles. A free-market zealot believes that by selfishly pursuing his own interests he does the greatest good for his fellows. That he is a Baptist or a Presbyterian or whatever on Sundays doesn't change the fact that his religion is commerce.
A person's religion consists of the core beliefs by which they direct their lives. If we think of beliefs in terms of a painting or drawing, what is absent is as important as what is present. Present in Christianity, Islam, and Judaism is the omniscient, omnipotent creator. That being is not present in the beliefs of the Atheist, and is a silhouette in those of the Agnostic. But the presence or absence informs and shapes the rest of the picture.