@Dichanthelium,
Recall that I said, "in the broadest possible sense, religion is humanity's desire to explain the world about them, explain the origin, maintenance, and future of their world and themselves..." What I mean by this general definition is
religion is the traditional pursuit of knowledge. Since all pursuits of knowledge have at some point in time been considered the ken of religion, in the broadest and most general possible sense of the term, all human knowledge can be considered religion.
Now a possible specific definition may be that
religion is the propagation of theology, that is, that thoughts that emerged from the body politik of a given religion can also be considered themselves commentaries (and thereby new ideas) on said religion. This is certainly the case with Vedanta
darshana, and since these commentaries are what's necessary for the survival of a religious tradition, then a string of commentaries that lead out from a given religious tradition can be considered said tradition's self-propagation through the years. Now from Greek (religious thought) arose the tradition of
Eleaticism, from which Socrates derived much of his conceptualization; and from Socrates Plato, and from Plato Aristotle; the body of thought of these men is the earliest occurence of what we would call modern
philosophy. The enduring chain of commentary--promulgation of theology--thus ensures a religious connection in the chain of philosophy; and since modern science developed from Bacon's naturalist interpretation of philosophy, the chain of commentary, the promulgation of a given religion, remains. Thus one can say that science and philosophy, under this definition, are merely the modern extensions of Greek religion. Needless to say, this conclusion is absurd, which means that the idea that religion is a chain of commentary or propagation of theology is a wholly inadequate definition of the distinction between religion and other modes of thought.
Therefore, I believe that the best definition of religion is a boundary definition, defining both what it
is and what it
isn't. A good definition of religion might be, "a system of philosophically interrelated concepts predicated on the acceptance, by faith, of said system, in spite of inherent methodological weaknesses." But then that has an overly fundamentalistic view of religion--what about enlightenment experiences? And doesn't that mean that no religion is good philosophy? Which is bad since I am convinced there are philosophical principles underlying good religion.
What religion
is is precisely the series of things that Judaism, Christianity, Islam, Zoroastrianism, Sikhism, Jainism, Hinduism, Buddhism, and paganism have in common. But the only things they have in common are traditions of commentary (which we have ruled out as being a definer of religion) and an appeal to a higher entity (which crops up in metaphysics as well!) But then there is a
third thing religions have in common...modes of prayer. All religions have a mode of prayer, prayer or meditation or the recitation of koans and mantras, but no other system of knowledge does. Thus I think the best possible limiting definition of religion is "a system of thought involving a mode (or more) of prayer".