The Seven Dimensions of Religion
As defined by Ninian Smart in "The World's Religions", 1989
These are the (very smart) Mr. Smart's seven "dimensions" along which a religion can be quantified. It's to be understood that the more of these dimensions are present, and the more strongly, in any human system, the more it qualifies as a religion.
P.S. For memory purposes, these make a really nifty mnemonic: P.E.N.D.E.S.M. --> "Pretty Eternities Necessitate Doubly Expansive Secular Mentalities". Isn't that just classic?
I. The Practical / Ritual Dimension. Contains all of the rituals and practices, including praying, marching, taking holidays, waving flags, etc.
II. The Experiential / Emotional Dimension. Includes religious experiences such as visions, revelations, enlightenment, and general religious ecstasy -- The acute and earth-shaking, as well as the gentler, more mundane religious feelings.
III. The Narrative / Mythic Dimension. The "story side" of a religion; includes written as well as oral tales, formal as well as informal teachings, alternative histories, predictions, etc.
IV. The Doctrinal / Philosophical Dimension. This is the official, formal teachings which underpin the narrative/mythic parts of a religion, though it's important to note that the Doctrine doesn't necessarily predate the narrative. In Christianity, for instance, it was the existence of a cult following the life of Jesus, and their stories and rituals, which led to the formation of the Christian doctrine. Note that very few doctrines are actually static over time, though their authors usually like them to appear so.
V. The Ethical / Legal Dimension. The laws, formal and moral, that come out of any system. Runs the gamut from the extensive and complex set of enforceable Mosaic laws to the variable, unwritten set of Christian taboos.
VI. The Social / Institutional Dimension. This and the following dimension are the only ones that require a physical form, as opposed to the others which can be purely abstract. The Social Dimension consists of the formal organization, such as the Church / mosque / umma / Sangha; as well as other institutions which may come about as a result of the religion; for instance the Salvation Army and Hamas. This is the dimension of "how the religion works in people's lives".
VII. The Material Dimension. This dimension contains all the physical creations of a religion, including buildings and architecture, ikons, art, instruments of ritual, etc. It also includes natural features of the Earth which may be important to the system, for instance sacred mountains, holy ground, Jerusalem, etc. The objects of the material dimension may be stunning, elegant works of Art, or they may be very simple and plain (as with Mennonites); but the point is, they're present.
Want to have a little fun? Let's try applying this to, say, nationalism. We'll use American nationalism as our guinea pig.
Is there a Practical dimension? Surely; we have the Fourth of July and the Pledge of Allegiance, for starters. Experiential dimension? Check - as any teary-eyed immigrant will tell you in a heartbeat, there's a very strong emotional component to being an American. Narrative? Of course - our national history, or our version of it anyway, is taught to every child via a national public school system. Doctrinal? Yes; this would be our Constitution and related works. Ethical and legal -- of course; there are plenty of laws and morals that are typically and especially American (I would list some, but it would only lead to a rant. ;) Of course our country has its social institutions, such as welfare, schools, etc. And as for Material, need I even mention the Washington Monument and our routine pilgrimmages to Washington, D.C.?
So hey -- Americanism is a religion. So, of course, is Englandism and Sovietism and Chinaism and Marxism and all the other nationalisms; I picked on my country because I know the details best. And yes, even "anti-religious" systems like Marxism can test out as religions, if they have all the necessary dimensions (which Marxism does, but secular humanism, for instance, does not).
Smart's dimensions are an excellent way to denote which human systems bear the traits of religion; and sure enough, when one does, you can almost guarantee that the people involved in it will treat it just like people involved in any religion do. (It may sound odd to say that "Marxism is a religion", but anyone who knows a Marxist can quickly see that he/she acts just like most religious adherents do about their faith.)
He doesn't answer the question, here, about what a religion IS; but he does provide a very clean, useful system of determining whether something is functioning as a religion in any given society.