The claim made in Roy Clouser's book, The Myth of Religious Neutrality, is that all of life is religious, that is, that for every person, every part of their lives is affected by their own religious beliefs. In other words, there is no such thing as religious neutrality, as everyone has religious beliefs (whether they realise it or not), and those beliefs touch every area of life.
Now, such a claim, in addition to being somewhat ambitious, would also be vacuous and meaningless if – as many have argued – no clear definition can be formulated as to what constitutes a "religious belief".
Finding such a definition is the goal of Chapter 2 of the book, What is Religion?
In essence the definition is as follows: a religious belief is a belief about what is unconditionally and non-dependently real. This may be a "primary religious belief", about the unconditional and non-dependent reality itself, or a "secondary religious belief", about how other realities (including ourselves) relate to this unconditional and non-dependent reality.
This idea of something being "unconditionally and non-dependently real" (what Clouser also calls "divinity per se") seems to be present in all religious belief systems, covering
God, Brahman-Atman, the Dharmakaya, and the Tao ... the Nam in Sikhism, Ahura Mazda (Ohrmazd) in early Zoroastrianism or Zurvan in its laver development, the soul/matter dualism of the Jains, the high god of the Dieri Aborigines, the belief in Mana among Trobriand Islanders, Kami in the Shinto tradition, the Raluvhimba of the Bantu religion, the Void, Suchness, or Nothingness found in various forms of Buddhism, and the idea of Wakan or Orenda found among various tribes of North and South America. It also holds for the ancient Roman idea of Numen, for Okeanos in the myths of Homer, and for a host of other ideas (p.20).
Clouser's definition of a religious belief seems coherent to me (and he defends it at great length): it is simple, precise and seems to trace out a fairly tight circle around everything we would intuitively term a "religious belief". It does, however, include some beliefs we would not usually think of as "religious" (such as philosophical materialism). But as long as we are clear what is meant by the term, I don't think that should be too much of an issue (and if philosophical materialism possesses the core characteristic of all religious beliefs, then calling it a "religious belief" itself doesn't seem too outrageous).
Chapter 3 will look more closely at types of religious belief, before the substantial part of the book, attempting to demonstrate that all people have religious beliefs (that is, that all people hold to a belief in something that is unconditionally and non-dependently real) and that these religious beliefs have "the single most decisive influence on everyone's understanding of the major issues of life ranging across the entire spectrum of human experience" (p.4).