I. What is Religion?
Religion is an artificial social construct. While relatively harmless compared to other social constructs such as government, laws, borders, and fiat currency, it has nonetheless destroyed millions, if not billions of innocent lives and has enslaved most of the population of the Earth. While many atheists assume that religion was created as a means of social control, the fact that acephalous societies get along just fine today (at least, when governments are not trying to kill or oppress them) yet no human culture ever known has lacked religion, is a good indicator that the reverse is true: the original acephalous foraging societies ("hunter-gatherers") had religion, and only later did the development of agriculture and permanent settlements result in the invention of theocratic government, liturgical religion, and writing. In other words, religion wasn't invented to control people, but control (government) grew out of, and can be considered a specialized form of, religion. Likewise, since the first state-level societies were theocratic, and since the invention of writing coincided with the invention of state-level government and the invention of liturgical religion while the adoption of writing systems and of liturgical religion almost universally coincide (to the extent that writing has historically not penetrated into certain parts of the world even to which it has for millennia been readily accessible, such as sub-Saharan Africa, until liturgical religions began spreading into such areas), making liturgical religion and state-level political integration almost indistinguishable on either conceptual or historical level, the modern, artificial distinction between religion and "secular" government is thus an entirely semantic one. The distinction or separation between church and state is, therefore, in reality, merely a distinction between theistic religion and atheistic religion.
I.a. Divinization of Biology
The most important thing to understand about religion is that it is based in basic animal instinct. Religion is not the grandiose, uniquely human enterprise we humans so anthropocentrically assume it to be. Communal feeding behaviors borne out in human religion in the forms of Catholic communion or various Pagan feasts can also be seen in many other social organisms, notably predators such as wolves and lions; many social species also have their own bonding rituals and sexual mores; ethics such as those imparted by human religion are also commonplace in other social organisms, for example regarding proper conduct in combat or appropriatness of killing. To be frank, we humans simply give ourselves far too much credit when we imagine religion to be some sort of lofty state of belief or bonding that is completely unrepresented elsewhere in the Tree of Life. We humans simply are not that special, nor is this thing we call religion built upon peculiarly human foundations.
I.a-1.) Ritualization of Nature
Religion, as a concept, is, first and foremost, a system of ritualizing the more basic, biological components of our lives and spiritually legitimizing (psychologically justifying) those basic animal behaviors noted above. Thus, religions often have rites related to birth, onset of sexual maturity / puberty (rites of passage), mating (marriage), reproduction, and death, as well as rituals concerning the changing of seasons (holidays) and the aforementioned communal feeding behaviors (feasts, communion, saying grace), the latter two of which are often intertwined due to humans being omnivorous organisms whose food choices and food-getting strategies change according to seasonal variables, with seasonal feasts featuring different foods depending upon the holiday (fruits at the Vernal Equinox and Summer Solstice, grains at the Autumnal Equinox, and meats at the Winter Solstice, for example, whence we get such traditions as the Yule/Christmas ham).
I.a-2.) Specism & Anthropocentrism
Many religions also feature a belief in human uniqueness or human exceptionalism, and are often anthropocentric in the extreme. This is understandable when one considers that sexually reproducing animals such as humans cannot afford to waste resources on attempting to mate with organisms so genetically different from themselves as to prevent successful reproduction, or on competing with individuals too genetically different from themselves to be concerned with having to contend with for mates. It is for this reason that humans very often consider social relationships with non-humans to be less meaningful and why many religions proclaim that human lives are more important than the lives of other animals, with some religious folk even going so far as to classify “humans” and “animals” as entirely separate categories of living things. This means that when some Christians and Muslims claim that humans are not animals, they are making a self-defeating argument; it is basic animal biology which impels us to believe in biologically invalid concepts like “kinds” or “species” and in the uniqueness of our own so-called “kind”, and precisely this way of thinking that best proves how un-special humans are. If humans were truly more spiritually aware, enlightened, and connected than other organisms, we would have abandoned ideas of separate “kinds” or “species” long ago, along with our primitive belief in absurd notions such as human uniqueness.
The best religions, such as Native American tribal/folk religion and neopaganisms such as Wicca and Druidry, are consciously aware of the animal nature of this thing we call “religion” and therefore seek to be much more geocentric (or at least more biocentric) and much less anthropocentric than other religions.
I.b. Projection, Theory of Mind
I.c. Fate, Destiny, and Determinism
In Understanding Human Nature with Steven Pinker - Conversations with History (15:18), Steven Pinker states that “when people reconstruct their autobiographies, they're much too apt to impose a satisfying narrative arc on it and to have everything foreshadowed by early events.”
Humans are one of a small number of species that have evolved to relay information from one individual to the next (Crows are another, as are Howler Monkeys and possibly also cetaceans). We are in fact such specialized vocalizers that our communication is not only informational like Crows and Howler Monkeys, but has evolved to include vocalization for aesthetic purposes, like the communication of the passeriforms (excepting, oddly, Crows, whose communication seems strictly informational and utterly devoid of aesthatic affectations). We evolved story-telling: The conveyance of information presented in an aesthetically pleasing framework; it is probably no coincidence that the most enduring stories are epic poems which were originally recited against a backdrop of musical accompaniment. Due in part to the psychological conditioning of hearing these stories during our most impressionable ages, we have a natural desire to place our own life stories into aesthetically pleasing frameworks with sensible narrative structures. We want to believe in omens and portents because this gives narrative cohesion to the events in our lives. When in our youths we hear legends, myths, folktales and faerie-stories, tales of angels and demons and giants, this tricks our young minds into thinking that we should grow up to interract with such beings; many adults in the U.S. still have a nominal belief in angels just as belief in fair-folk is still common in Europe; Christians believe their own lives to be epic struggles for their immortal souls, with every action either pleasing Yahweh or benefiting Satan as a part of some test to determine their eternal fate, and that this is in-turn relevant to the ultimate fate of the universe as all of Yahweh's "saints" shall one day fight against Satan and his legions in the Apocalypse. Religion provides us with a means of making our own lives seem more epic.