The common factor shared by all religions, according to Roy Clouser, is belief in something that is "divine per se", that is, something that is "unconditionally and non-dependently real". Chapter 3 of his book, The Myth of Religious Neutrality, looks at this definition in some more detail. Two things are worth noting.
First, a consequence of the definition is that something that is "divine per se" (or simply "divine") must necessarily exist.
The sum total of reality, no matter how that is understood, would have to be divine either in part or whole just because there'd be nothing else for it to depend on (p.57).
This means that any coherent understanding of the whole of reality is "religious", in the sense that it involves belief in something that is "unconditionally and non-dependently real".
The second thing to note is that there are various ways in which the divine and the non-divine might relate to each other. It might be the pagan dependency arrangement, under which
there is only one continuous reality, a part of which is the per se divine on which all the rest depends (p.44).
Examples of this would be nature religions, most polytheistic religions, and materialism (the belief that matter and energy are "just there" and that everything else in reality depends on them). Or the divine and the non-divine might relate through the pantheistic dependency arrangement:
Instead of locating the divine as a subdivision of the one continuous reality, the pantheistic belief is that whatever we experience as non-divine reality is in fact a subdivision of the divine reality, which is both infinite and all-encompassing (p.48).
Examples would be Hinduism and Buddhism. Or the third way in which the divine and non-divine relate could be termed the biblical dependency arrangement, under which
the divine per se is not part of the universe nor is the universe part of the divine; there is a fundamental discontinuity between the creator and all else which is his creation (p.50).
Examples of this would be Christianity and Islam.
Having now established that all coherent belief systems about reality are "religious", the next step in establishing his thesis that all of life is religious is to show precisely how it is that religious beliefs shape how people understand anything and everything. This is the subject of chapters 4-6, on "Theories".