Metanexus has posted an excerpt from Clouser's excellent book, The Myth of Religious Neutrality. The excerpt includes Clouser's novel view of what constitutes religion. Clouser defines religion as the belief of something which is not dependent on anything else. Therefore, since the materialists believe that matter and the laws of nature are not dependent on anything else, by Clouser this would be fall under the category of a religious belief. Similarly, the gods of the Roman Empire would not be considered a religious belief because the gods (at least most of them) were dependent on something else for their existence.
In memory of Michael Crichton, I'm linking to two of my favorite essays of his: