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Religion is about our 'human spirit'. This may mean a simple shared philosophy, such as a 'team spirit' familiar to participants of successful sporting and military activities, or a highly complex set of religious beliefs, sacred laws and temporal regulations about how 'we' may conduct ourselves and how 'they' -the rest of the world - is (or should be) properly ordered.
Religion is a cultural manifestation that may have had a role in the survival of human groups and nations. Traditionally most religions were enforced, and people who failed to conform to religious rules (apostasy) or who expressed doubt (blasphemy) or interpreted things differently (heresy) were severely punished, often by death. Generally religion requires its adherents to support each other, and either enables or requires enemies (including the 'internal' dissidents just mentioned) to be suppressed or killed 'lawfully' within a general philosophy of 'divine justice' or some specific pronouncement of 'justified war'.
Religions develop depending on the way of life: Gathering and field work once were female activities and are highly dependent on the soil, leading to earth godesses. On the other hand, hunting and pastoral work were reserved for men, causing their gods to be male. These gods are often associated with the weather or the sky.
In much of the western world, where Christianity is dominant, a common mis-conception of religion is simply that it involves a God, or god-like figures such as emperors, royal households or priestly organizations. Since such people are supposedly 'appointed by God' they are generally assumed to be divinely authorized to rule, utterly incapable of error and so worthy of praise, love and devotion.
This westernized view is however, extremely incomplete. It excludes many human activities that generally fall under the heading of "religion" and distorts the spiritual beliefs of many people in other religions.
For example, Buddhism is a religion without a God. Though many forms of it have imagined the Buddha as a Christ-like figure, other Buddhists think of him as an historical figure -- not to be worshipped, but respected as a man of insight who set an example for others to follow.
Afro-Cuban religions like Vodoun include loa, powerful beings who are not gods, and are not worshipped, but are dealt with as we deal with our fellow humans. Believers allow these beings to possess them, under certain specific and controlled circumstances, in order to converse with them and receive advice. Quasi-religious practices, such as Taoism, or certain tantric schools, are harder still to classify in any easy way.
So before we can arrive at a toolbox for building a constructed religion, we must arrive at a reasonable definition. I propose the following as a reasonable definition of a religion: A set of metaphysical beliefs that describe the sort of existence we live in, suggests a goal its adherents wish to attain, and prescribes a method whereby that goal may be obtained by the individual.
Forms of Religion 
Religions take many forms and variations. Don't let difficult words (and -isms) scare you! Some of them are described below, but the most common forms are atheism (believing that there is no god), agnosticism (unsure if god exists or not), monotheism (one God), polytheism (many gods) as well as monolatrism, polylatrism, henotheism, kathenotheism, deism, idealism, and animism.
Atheism and Humanism 
Atheism is non belief in Gods and Goddesses. ( Theism is belief in one or more of these. "A" is from Greek, meaning "not","without"; hence "Atheism" is "Without Theism".)
Atheists hold that ideas of Deities are merely human constructs.
Humanists omit belief in the Supernatural and concentrate on human life in this world. Some humanist groups have trained officiants to lead weddings, funerals, namings or other life-passage ceremonies if individuals need them.
All ethics are seen as derived from human understanding of actions and their consequences. Laws are not seen as given by Deities but arise out of society's consensus about how actions should be regulated.
Those who find Theism unbelievable of course share the common goals of all people who want to live in societies that promote happiness and security.
View that God’s existence is unprovable: the belief that it is impossible to know whether or not God exists. Agnosticism is rarely the basis for a whole belief system but is commonly associated with atheism or skepticism.
Monotheism is the belief in one single god exclusively, and that no other gods exist. This is the form professed by most, if not all, denominations of Christianity, Islam, and Judaism, although Trinitarian sects of Christianity are sometimes construed — generally not by their own followers — to have a "pantheon" of three gods (the Trinity).
Polytheism is the counterpart of monotheism and holds that multiple gods (forming what is usually called a pantheon) exist. Most of the religions now regarded as "mythology" and most neopagan religions embrace some form of polytheism.
(Note: Different definitions of Monolatrism appear. The definition given here is not the only one.) Monolatrists worship only one god, but believe other gods exist. They believe that the other gods leave non-worshippers alone. It has been suggested that early Judaism was a monolatrism based on alleged references to other gods in sacred texts.
(Note: Polylatrism is not a word in real usage. It is used here for convenience and because the word just makes sense.) Polylatrism implies that a people worships a sub-set of gods exclusively but believes in the existence of other gods. They believe that other gods leave non-worshippers alone. Many neo-Pagans adhere to something akin to polylatrism, but the closest thing to a historical example we have might be the ancient Celts, which evidence suggests held a policy of "you worship your gods, we'll worship ours".
Kathenotheism is a hard concept for many to understand, but might be best described as worshipping a Divine Office instead of the god holding it. Kathenotheists might believe there is a series or cycle of supreme deities, and worship each one for only as long as they hold the power of supreme god.
Henotheists worship one single god while not denying the existence of others. The difference between henotheism and monolatrism is that henotheists seem believe that the non-worshipped gods have power over those who don't believe in them.
(Note: Deism has had many forms over the years. This is just one.) Deists assert that a power created the world but abandoned it, or at least doesn't interfere with it. Some forms of Deism say that in creating the world, the creator became part of it; this is a kind of pantheism. This form of religion appeared heavily in the thinking of many Enlightenment era philosophers, though not as of yet in a major religion. Famous Deists include Thomas Jefferson and Benjamin Franklin.
Idealists, in this sense, worship or strive toward a creed, philosophy, or perfect ideal and might be considered a secular religion. Examples include the North Korean philosophy of Juche and arguably many forms of Buddhism.
Animists believe that a soul or other supernatural power is in every single object. Mostly associated with pre-agricultural or hunter-gatherer cultures.
Manism is based on the belief that the deceased, often family members, mythological ancestors or saints, have a continued existence and/or possess the ability to influence the fortune of the living. Manism is most frequent in settled cultures.
Concepts of Death 
This is a belief that after death, one will be born again in some form. There are many belief systems around this idea. A Southeast Asian folk tradition holds that one is reborn into the same family, so that you might be your Great-Aunt Edith.
The most famous belief system pertaining to reincarnation is that of the Hindu Brahmins, which was exported to many lands and forms the basis of many New Age philosophies. In Brahminism, acceptance of one's station in life is necessary to improvements to one's status in future lives. Thus, a wealthy member of the nobility deserves his good fortune because of his actions in past lives, and the low-caste and untouchables similarly deserve their fate. The ultimate state, nirvana, is an extinguishing of all personality and desire.
Not all who believe in reincarnation share this philosophy. Gnostic Christians, Druids, and Native Americans among many others have their own ideas of what it is like.
Nothingness after Death 
This is the belief that nothing happens after death, except the bodily functions and consciousness cease. Most people would agree that the lives of the more charismatic ancients are recorded in human fable, religious tradition or vernacular (written) historical records. It is thus common for humans to achieve a sort of vicarious immortality simply by leading a full life which is interesting so capable of being 'bought alive' in the minds of our survivors and their students.
This is the belief that some kind of world (an ideal heaven, limbo, underworld, or torment in hell) exists beyond the grave. It often provides at least two possibilities of happiness and torture based upon one's performance in life, or specific actions in one's life. This can be a powerful control myth for young impressionable warriors who may face death on the battlefield, and who must obey orders without question if their army is to be successful. It is also a useful mechanism for instilling 'ethical values' and promoting 'righteous behavior'.
The underworld, like the Ancient Greek Tartaros, might also be akin to nothingness; the dualism of Heaven and Hell is actually a quite recent invention. According to the Romans and Greeks, everyone —except for those who had directly committed a crime against the gods or those who had been really virtuous heroes— would go to the Asphodel Meadows, a vast land with nothing in it. It might be compared to Purgatory, although the Asphodel Meadows were permanent.
Spiritualists believe that the next life is a place where one goes on learning and having a variety of experiences just as we do during our lifetimes. Many ancient peoples including the Egyptians conceived of the afterlife as being like Earth but without life's problems.
Although often overlooked, ritual holds an important place in human behavior. Ritual means repeating a sequence of activities in a routine (formulaic) way. Ritual may be something trivial, such as wearing 'lucky' clothes for certain important occasions such as job interviews, or extremely important aides to memory, such as the checks performed by a pilot before attempting to fly an airplane. Religious rituals generally are performed to make sure that all participants are 'of one mind' - that they have the same understanding of 'right' and wrong' behaviour.
Some rituals may seem bizarre, but hold an important 'survival' truth. The Pagan belief in sowing seed in the nude is an example. Without clothes, the farmer is sensitive to the air temperature, wind speed, soil condition, moisture content, presence of predators and so on. The 'comfortable' condition (in Southern Europe, where the practice emerged) just happens to be about ideal for seed germination, and so people who performed this naked sowing ritual also tended to have better harvests than those who did not!
Most religious rituals tend to be simply routine reminders of our ancestors successful seasonal habits. The feasts of special food (for example the Western Christmas or Saturnalia, which is a winter solstice feast that tends to prevent depression) or fasts or absence of something familiar (Notably in early spring, when food is scarce, and we need to lose fat gained during winter) are common manifestations of formerly successful seasonal survival strategies for our ancestors, who lacked central heating and refrigerators.
Most important however (both for a human survival and for our sheer enjoyment of human life) are public or religious celebrations of life's major events - birth, academic achievement, maturity, marriage, child rearing, sagacity and death. Most of these 'life changing' celebrations have become adapted, stylized and documented to reliably provide example, advice and guidance to attendees who aspire to achieving such goals.