A religion is a systematic set of dogmas and, usually, rules for behavior, that involve some supernatural explanations for aspects of life as we know it.
The system of beliefs and rules are called dogma; religions vary in how much dogma they include and how strictly they define it and enforce it.
Major features
Most of the world's religions have several features: mythologies about creation (some admit they are, some don't) and man's place in it; answers of some sort for life's mysterious questions (what happens before life/after death or whatever); a requirement to have faith in the religion's ideals; and some sort of guidance (rules) for how to live in the world so defined.
They often also tell tales of what will happen at the end of the world, as well. Another almost universal feature is some form of proselytising, whether by personal contact or by print.
Range of religious perspectives
Some religions are so bereft of silliness and dogma that they might as well be called philosophies. At the other extreme, some are rife with superstition, myth taken as fact, and harshly enforced rules.[1] People who are not their adherents call them "cults", those who share their dogma (the "faithful") think they are the ultimate truth, even above the evidence of their senses.
Religion and the supernatural
The supernatural aspects of religion "evolved" socially as part of how a people would deal with unanswerable questions, and the rules often developed as clumsy or elegant solutions to how to live with others and to survive as a people. Though these rules may have been useful in the past they clearly have little to tell us about life in the twenty first century.
Modern approaches
Nowadays, people frequently create a "personal philosophy" that is a contradictory mixture of the religious, philosophical, and moral concepts that they feel is appropriate for them.
Of course, there are sadly still many people who insist that every teaching their religion promotes is the absolute truth - these people are called fundamentalists.
Religion and science
Historically, due to the aspect of religions' needing to "explain" natural phenomena, as science has improved its explanations of them, the two fields have experienced friction. In some - perhaps rare - cases religions have encouraged naturalistic explorations and explanations, but at the other extreme, they have severely suppressed these attempts. It is, however, difficult to imagine a religion which does not, at some level, rely on supernatural explanations. The saner ones just restrict their imaginary friends to regions of space science has not explained yet.
Reasons for rejecting religion.
There are a number of reasons why sane people reject religion
religion is Anti-intellectual
All religions have in common that they are faith based. People are taught to believe claims from some iron age text or self-proclaimed spiritual leader, instead of relying on their own senses, evidence and critical thinking.
Indeed, as Christopher Hitchens said: "What can be asserted without evidence can also be dismissed without evidence."
Religion is Anti-scientific
Some religions try to appear "science-friendly" by avoiding statements that clash too obviously with modern science. This however only obscures the fact that the very concept of "faith" is inherently anti-scientific and cannot be tested by using the scientific method.
As all religions rely, at some point, on faith it is easy to show that they are not scientific. A key test of whether or not a hypothesis is scientific is the question of falsifiability. What evidence would the hypothesis' supporters accept as being able to falsify the hypothesis? By definition, as religions are based on faith and not evidence, no evidence can be presented which would persuade a religion's followers it is wrong - and consequently no religion can be scientific.
Religion needs the God of the Gaps
Arguments based on "First causes" are ill-conceived attempts at "proving" the existence of a god - of whatever type - by suggesting that the universe must have come from "somewhere", or that something must have existed "before".
However science suggests that time and space are intimately linked - there was no "before" for the god to exist in. To quote Stephen Hawking: "It's like asking what's north of the North Pole. It's an ill-posed question."
Religion needs a creation
The very concept of creation depends on time. Something that did not exist before a given point in time comes to existence through the act of creation and exists from that time on. The universe clearly cannot have been created in this sense, because the universe is spacetime and thus there is no time outside of the universe itself.
God vs. religion
It seems to be clear that a God in the sense of a miracle-working, interventionist deity does not exist - as no evidence of his miracle working, interventionist activity can be found. In this case absence of evidence is indeed evidence of absence.
It cannot be excluded that some kind of completely non-interventionist "god" (whatever that is supposed to be) exists outside of our universe. But even if that were the case, it follows that a non-interventionist god cannot interact with the physical world in any way. In particular, he cannot write books, nor can he put thoughts (which consist of electric currents in the brain) into people's heads. This shows that, whether or not a non-interventionist god exists, religion is wrong.
Religion and Soul
This is said by many religions to be some kind of spiritual entity that every human is supposed to have, and which is supposed to survive when the physical body dies. But since this thing is non-physical, it again cannot interact with the physical world without violating basic science. So we have the same situation as with "god": a "soul" could hypothetically exists, but it can have no connection with the physical world.
NOMA
There are some attempts to rehabilitate religion by saying that it has no scientific implications. The idea is that science and religion operate in fields so distinct that each one cannot examine, or comment on, the other.
This attempt is called NOMA, and it is mistaken. First of all any interaction of a god with the physical world world would most certainly be within the scope of science. The only way to remove god from the physical world is to assume some sort of deist god, and that is certainly not the god of most religions.
Furthermore the supporters of NOMA seem quite coy about where the NOMA line should be drawn. Religious beliefs are the basis of creationism, intelligent design and theistic evolution. While many science-friendly NOMA advocates happily attack creationism they then hold the NOMA shield up to defend other (their own) religious beliefs - a position which could seem a little intellectually dishonest.
Religion and Morality
One of the things which many religions claim to possess is the authority to give eternal moral (or ethical) guidance to their followers. However, the fact that religious beliefs have frequently been important factors in causing or exacerbating divisions between peoples - sometimes even resulting in warfare - rather devalues religion's claim to the moral high ground.
Furthermore, it is quite clear that religion's ethical advice is not eternal, as many religions' ethics evolve with society. This is clearly not a bad thing, but it removes religion's ability claim absolute moral standards or to pontificate for all time about morality.
The question of respect
Finally we have the suggestion that religious ideas should, for some reason, deserve a special measure of respect not given to, say, Homeopathy, or UFO conspiracy theories. (The concept of NOMA, mentioned above, is part of this.) But why should religion deserve this special status of being "respected"?
If religions can make a forceful, rational intelligent case then they would be respected for that. If they are unable to present a forceful, rational intelligent case, then whatever respect they get should be based on their failure to do so.
We are also sometimes told that we should respect religion because some sincere, intelligent people believe in it. This is simply repetition of the argument from authority fallacy, and may be dismissed without further concern. Indeed one could quite easily produce a list of sincere intelligent atheists which would equally prove nothing. The question is where is the evidence?