Definition

Religion is a belief system that is based on faith rather than objectively verifiable criteria. Most religions share most of the following characteristics:

• belief in one or more supernatural agents that take interest in and affect our fate;
• worship of and prayer to these supernatural agents, in hopes of bringing about events beyond human or natural means
• purported answers to some fundamental questions of human existence, e.g.
  • How did the world around us come to be as it is, and especially how humankind?
  • What happens to us when we die?
  • Why do good/bad things happen to us?
• a moral code, i.e. a set of rules for adherents to live by; and
• a culture and community of adherents, based around the religion.

Prehistorical origins

Early humans, not privileged with the knowledge and power humankind has meanwhile gained from science and technology, found themselves in a bewildering world whose complex correlations it could often not fathom. At the same time, the early human was as curious as modern humans are. One could without condescension regard them, intellectually, like a child: Somewhat helpless and fearful, full of wonder and brimming with questions.

Palaentologic research into early humans and anthropological research into primitive cultures still in existence today in far-off places agree that each society will tend to create a religion. Many religions are inspired by a society's immediate surroundings: Native Americans had myths surrounding world-creating bears, crows or other animals. They held that spirits inhabit not only all living things but also non-living, such as mountains. The Inuit of the Arctic have a rich mythology that prominently features seals and other sea-dwelling creatures. And so on.

The following is a hypothesis! I offer it not as the product of peer-reviewed research but because it makes sense to me.

If there were no God, it would be necessary to invent him - Voltaire

My hypothesis is that Voltaire has hit the nail squarely on the head.

Faced with many unanswerable questions, humankind needs simple, authoritative answers.

Humans are ill at ease with unanswered questions. They make them feel stupid. In this situation, any answer is better than no answer.

Consider yourself in the situation of a parent whose child has just discovered the word "why." "Why is the sky blue," she asks? "Because God made it that way. She likes the color blue." There: A simple and at least temporarily satisfying answer to a difficult question.

Faced with the vagaries of the elements, humankind needs a strong helper and protector.

Hunters depend on the movement of game; farmers depend on rainfall. Humans cannot control rainfall to this very day. Again, an unsatisfying situation. Imagine the horrible feelings of depression and incompetence of an agrarian tribe, waiting for rain with nothing to do but hope. There must be something you can do! Enter the spirit/deity who can be swayed with sacrifices, moved by prayer or tickled by worship. Presto, a wealth of activities to give the rain-waiters something ostentatively productive to do. While rain dancing may not help objectively, it makes people feel better.

Faced with the dreadful concept of death, humankind needs consolation.

A child loses his favorite grandparent. One day he was there, smiling at the youngsters and telling stories. The next, he's a lifeless hulk hurriedly carted off to burial or a funeral pyre. "Where's Grandpa?" "He's in the sky now, watching over us." That's tremendously more comforting than "He's wormfeed now, and his nutrients are being recycled to make flowers and grass."

Later, that same person is faced with his own upcoming death. Without consolation, he may despair. But in the knowledge that he will continue to exist on another, better plane, he will not fear death as much.

Feelings of inadequacy and despair, though "only" in the mind, are a deterrent to human survival. Unchecked, these feelings can paralyze a person, rendering him incapable (or at least unmotivated) to do what's necessary for survival. Inventing supernatural beings and processes is an excellent stopgap solution!

As mentioned before, it's hard to find a culture that hasn't invented some kind of religion. Recent research seems to indicate that there's a part of our brain dedicated to religion (http://atheistempire.com/reference/brain/). There can be little doubt: Religion was such a strong survival trait for the human race that it's been selected by evolution and wired into our brains.

From Stone Age to Modern Age

In a prehistoric or primitive context, religion apparently helped humankind with some troubling problems. In a tribal context, religion was often the office of a witch doctor or shaman who practiced rituals and teaching "professionally," often full-time. Such an office was associated with some political influence. But I believe it wasn't until the advent of larger, city-dwelling agrarian societies that religion grew into a political force.

Religion is regarded by the common people as true, by the wise as false, and by the rulers as useful. - Seneca

Rulers who could claim the support of their spirits/deities, or to be acting on their behalf, found a religious populace easier to rule. At the same time, religion for such a society expanded into a job for an entire clerical cast. While history shows instances of worldly governments and the clergy working at cross-purposes within a state, both were usually smart enough to realize and draw benefit from mutual cooperation. King Henry IV's Walk to Canossa is an interesting anecdote of a power figure humbling himself to regain clerical favor.

Sometimes, secular and clerical leadership were combined, as in the Hebrew kingdoms of David and Solomon. More often, the clergy maintained its own hierarchy independent of secular leadership. In either event, religion became a political force, used to enforce laws, unite people and rally armies.

More

...as time and inspiration permit. Or is anyone else willing to pick up the baton?