John Loftis, at his blog Debunking Christianity, lists ten objections, concepts, or topics that seem to be raised most often by atheists in debates with theists. Here's the eighth (my response to the earlier objections can be found by scrolling down the page):
8. Atheism is not materialism. Materialism is a scientific doctrine, while atheism is a stance on the position of gods. Arguing against materialism is not going to make the case for theism.
Materialism is not a scientific doctrine. It's a metaphysical doctrine adopted by many scientists as a methodology and by many others as a metaphysics. Loosely, it's the belief that all of reality can be reduced to material particles (and their energy equivalents). On materialism there are no immaterial entities like minds, souls, or spirits.
While it's true that atheism does not strictly entail materialism, most atheists are in fact materialists. They see it as coherent with their belief that there is no God that there is also no soul (mind) and no immaterial beings nor immaterial entities of any sort.
If materialism is false or can be shown to be doubtful, it would be very upsetting to most atheists because it would call into question a major chunk of their worldview. That's one reason why there was such a backlash by other atheists against atheist philosopher Thomas Nagel's book Mind and Cosmos: Why the Materialist Neo-Darwinian Conception of Nature Is Almost Certainly False . Nagel argues in the book that materialism simply cannot explain conscious experience, values, meaning, and the origin of life. His fellow atheists were very upset by this heresy since for most of them materialism stands in relation to atheism pretty much as belief in souls stands in relation to theism for the theist. You can have one without the other, but it's awkward, uncommon, and unsatisfying.