I never did like saying it that way because it's not in the affirmative. It doesn't tell you what they believe. It tells you what they don't believe. The affirmative way of saying it is that atheists only believe in a material existence. That there is nothing beyond the material world. Which is where it gets hard for them. Of course there is something beyond the material world. It would be idiotic to believe there was nothing beyond the material world. Science says there is something beyond the material world. Science just can't define what it is. That is the realm of philosophy.