I define religion as "any strongly held set of codified beliefs." What does this mean? Well "strongly held" might seem obvious. In this I mean that the beliefs are, in some way, core to the individual's identity of self. I should point out here that one can belong to more than one "religion" at a time, very few are truly exclusive. "Codified" refers to some standard set of beliefs. This might be the Apostle's Creed, the Republican Party Platform, Libertarianism, Feminism, Hinduism, or Evangelical Christianity. Each of these has an outline that defines the belief system of its members.
A religion, therefore, has a level of homogeneity amongst its members. They often identify themselves by the religion even moreso than by their own achievements. "I'm a Christian" or "I'm a Liberal." Rather than define their own specific beliefs or where they may, or may not, agree with a specific label, they identify with the label. This can cause a great deal of psychological stress should the label they use to identify themselves become altered in the public perception.
An excellent example of this is the United Church or Anglican Christian who has suddenly found themselves a significant minority when it comes to the word "Christian." The label has been altered in the public eye because evangelical and coercive sects have become the majority. Christian, in the public lexicon, no longer refers to the core values of the average Anglican or United Church member.
Another example has been the re-branding of the Alliance and Reform parties in Canada. These extreme right wing parties were doing very badly until the Progressive Conservative party of Canada was almost drummed out of Parliament for various scandals. They then re-branded themselves with the Conservative colours and name (The Conservative Party of Canada) and stole the election after a similar Liberal scandal. The problem is, they have continued with their extreme right wing policies to the point where old-school Conservatives are embarrassed to use the term. Once they identified strongly with the word but now it has taken on a completely different meaning. (Sadly, they were also mostly those who voted for them thinking they were voting for their old party.)
The point is, a "religion" is strongly held in that it is often one of the things that we use to define ourselves as we integrate the codified beliefs into our personality. It helps us to feel that we are part of something greater than ourselves, gives us purpose and helps us to identify like-minded individuals. Psychologically this is an important part of our development and it is often easier to work with a predefined set of beliefs than it is to deconstruct ourselves and figure out what we are when we are not identifying as x or y. This often happens in middle age. Some individuals become so comfortable with themselves that they are able to deconstruct and analyze themselves as individuals. Others become comfortable and reinforce these predefined beliefs into a hardened and impenetrable structure.
More and more, though, we are seeing people trying on various ideas of self at earlier ages. They reject the religions of their youth and seek new structures, often confusing Religion with Christianity. Is the "Goth" subculture really any less of a religion than anything else? Their shared belief structures create an homogeneous whole often through emulation of one or more iconic individuals (what we might call "cult of personality" or "prophets" in ecclesiastical terms).
It should be pointed out, though, that even amongst spiritual religions, Christianity is a minority. Most religions do not have internal exclusivity. A majority of people in Japan, for example, consider themselves both Shinto and Buddhist. In the Old World it was considered polite to make sacrifices to the local Gods when travelling, regardless of what Gods you personally worshipped. If you were a Priest of Hern in Brittania, you would still throw incense on the coals as you passed a shrine to, say, Diana, while walking the streets of Rome. Even Orthodox Rabbis I've known do not feel that they have the right to impose their Law on others. The seven Laws of Noach which everyone must observe to live in Hebraic lands ask only that you respect their God, not that you must conform to their method of worship.
And this brings me to the most interesting aspect of Religion in the West. We've managed to make almost all of them confrontational. The Christian Majority believes that all others MUST conform to their way of thinking. This has tainted Political Parties, polarized our countries and made everything confrontational. The Right vs the Left has caused them to drift so far apart that they will shut down a proposed law just because the other team thought it was a good idea. It may have BEEN a good idea, but this level of religious "rightness" prevents them from seeing it as anything BUT adversarial.
I believe this is because Christianity adopted the Zoroastrian idea of an adversary. Whatever you believe, someone is trying to work against you, destroy you, and make you suffer for eternity. Even those of us who have escaped the primary Christian programming cannot escape our society's adversarial nature. Materialists take every opportunity to proselytize their religion even in the face of possible doubt. Atheists violently oppose the idea of God believing that all concepts of the Divine revolve around a voyeuristic vending machine sky daddy. They are so ready to argue and defend their position that they cannot even HEAR the words that are actually being spoken.
They do not tend to be self-aware though. Just as a Christian can point to their Bible and have a huge percentage of people on the planet agree with them (about 30% for New Testament, around 40% for the Old (Tanakh)) so too does a materialist rely on a consensus when they believe in things they cannot see. I cannot see an electron. I have a friend who works with them. If he shows me a plate and says "look a picture of an electron" I can either believe him, or not. He may be lying to me or be outright wrong for all I know. We read things in scientific books, but we are still putting faith in the people writing them.
When we get into more complex systems there are people who believe in M-Theory, and people who believe in Quantum Loop Gravity theory and people who believe in other theories. They are all certain that they are right, just as certain as most other religious adherents. Some conform to both a scientific and a spiritual religion seeing the beauty and perfection of their work as proof of a sublime consciousness in the universe.
In the end, Religion is a process of the brain whereby we believe in certain concepts and "truths" based on an agreement with others along the same lines. We integrate those beliefs into ourselves to the point where we begin to define ourselves by that structure, label and code. These beliefs then become the foundation for all of our motivations, actions and activities in life. If we believe in universal love and compassion, we will try to help and heal others. If we believe in fear and adversarial theology, we lash out and become a threat to others. If we believe in helping the poor, either through compassion or by commandment, then we will make efforts to do so. If we believe in free market corporatism then we will do our best to maximize our own profits at the expense of others.
It is for this reason that a study of religion, both domestic and abroad, can give us significant insights into the behaviour and motivation of ourselves and others. Religion and Culture are very closely related, as we can see by the development of our divisive culture based on adversarial Christian values vs. more unified Asian cultures where there is greater acceptance for one's countrymen based on a non-exclusive cosmology.
I know it is common amongst Occultists to reject all "organized religion" and, by doing so throw out all aspects of codification, often turning to Chaos Majik or some other pseudo-chaotic approach. (Chaos Majik is JUST as dogmatic and codified as anything else by the way.) But the organization and structure of, say, the Roman Catholic Church exists in no other spiritual faith tradition. Even Judaism, which is often seen as the Father of Christianity (much to the Jew's chagrin) does not have even regional organization. Each congregation builds their own Synagogue and hires their own Rabbi. Shinto shrines register with a central registry, but they function on their own. Therefore I do encourage you to examine other religions, see what they really say for themselves and even if there is nothing there for you personally, you will have learned to understand another group of people in a way you never could before... and if we're all expressions of the same Limitless Light, then learning about your brethren in this way can only help us in our journey.