Religion: Intellectually, belief in the supernatural. Socially, institutions and practices based on belief in the supernatural.
Here I use "supernatural" in the obvious sense of: beyond or outside natural reality, that is, unlike divinities and angels, represented in ordinary human experience, and in principle accessible to all, and the foundation of all experiments and tests to check theories.
1.1. There are and have been very many religions, and it is difficult to identify what is common to them, but all seem agreed in the assumption that there is more than natural reality, and that the knowledge about this supernatural reality is somehow delivered by prophets, priests, holy men etc.
Some variants of Buddhism, for example, do not assume there is a god, but then they do assume there is a Nirvana and that one's unenlightened experiences are all illusion.
The writer of this dictionary rejects all claims to supernatural knowledge; does not believe in any god; and disbelieves the vast majority of all religious teachings and ideas he has been in contact with. (He also never had any religious faith, and was not raised in one.)
This does not at all mean that he believes there is not very much that human beings do not know, and quite possibly also that there is much that human beings cannot know, but it does mean that he believes such knowledge can not be found in Holy Books (excepting some teaching about ethics, which has nothing to do with the existence or non-existence of supernatural entities), and that he does believe that scientific realism is the best approach to real knowledge.
1.2. Five simple reasons for this firm belief in science as the method to reach knowledge are:
1. All religions I know of are full of provable falsities and provable nonsense, and have done much harm, which may have been also mixed with some good.
2. No God or angel ever appeared to me (other than in the delusive shape of attractive young women).
3. No presumed Holy Book whatsoever, in spite of its usual claims to contain the true teachings of the Maker of Universe, Infinite in Power, Knowledge and Benevolence, contains as much as a millionth part of the scientific knowledge found the last four centuries, since the firm foundation of the scientific method by Galileo and Newton.
4. Real science produces real technology that really works whether or not you believe it or can explain it, whereas real religions produced no technology other than new ways of deceiving the stupid and the ignorant, and no religion produced any technology that really works without faith in the religion.
5. All or nearly all of the faithful of all religions fondly believe the same about all religions other than their own: They disbelieve all of them (except perhaps on a dishonest and confused verbal level). I merely extend this from all-but-my-own to all.
1.3. Religion seems to be the price thinking apes have had to pay for becoming intelligent enough to formulate language and find the symbolical tools of abstract thought. It is not odd to make supernatural hypotheses if one has hardly any knowledge about nature, and has much to fear in it, and has the capacity to formulate infinitely many hypotheses to explain one's experiences. What is somewhat odd - supposing these thinking apes to be rational, as many of them believe they are - is that what is evidently mostly superstition, make-belief, wishful thinking, or inspired by fear, has been adopted and believed by so many. And what is rather frightening, besides showing something about the animal or beastly nature that is also part of human nature and showing somehing about the foundation of human societies and human groups, is that so many human beings have been murdered or persecuted because they had adopted the wrong superstition in the superstituous eyes of others.
2.1. Any human society (beyond minimal complexity) is based on shared beliefs, shared desires and a shared language. The beliefs involve what the world and human beings are like; the desires involve what the world and human beings should be (made) like; and the language makes it possible to share and discuss beliefs and desires, and come to agreements, and help cooperation and communication.
The beliefs and desires (or ends) that serve as a shared basis of agreements about what is the case and should be the case are conveniently and conventionally called an ideology, and any religion, like most more or less worked out political ideas and values, is in this sense an ideology.
2.2. It is not amazing that a group of thinking apes, that find themselves in a world they understand little of and have much to fear of, adopts some set of supernatural beliefs. Indeed, there are three kinds of fairly strong sets of reasons for doing so: First, the whole idea of a society is based on the idea of shared desires; second, wishful thinking works in the sense that it unerringly arrives at such beliefs as one desires (that often have much to do also with what one fears); and third because such supernatural things and forces as have been assumed tend to be based on easily comprehensible features of humans and human societies: Gods and angels tend to be much like humans, but more powerful.
The five simple reasons given above against religion as a method to reach knowledge and in favor of science seem to me quite strong against any specific religion, but there are four general arguments concerning all religions that also need to be listed and considered seriously, especially by religious believers, followed by a practical, practicable and moral conclusion about all religions and all religious believers.
3.1. Ockham's Razor says in Latin 'Entia non sunt multiplicandur sine necessitatem' which freely translates as: One should not assume more than is necessary to explain what one wants to explain.
This principle - that seems not to have been found literally in Ockham's texts, but anyway is as old as the 14th Century at the latest - can be argued in several ways.
One is simply that any extra assumption is an extra possibility of being mistaken, and of introducing a piece of fiction to account for some fact or facts. Another is by way of basic probability theory, in which it is a theorem that pr(P&Q) <= pr(P): Any conjunction of statements (including assumptions) cannot be more probable, and normally is less probable than any of the statements in the conjunction.
This accordingly also holds for the hypothesis of a god or gods to account for nature or natural things: That there is both a god and nature, is less probable, if god's existence is not provably logically or physically necessary, than that there merely is nature, even if we may not know enough about nature to explain everything or indeed much in it.
Besides, in the case of assuming divinities one does not just add another small possibility in which one may be conceivably mistaken: One adds a whole layer of complexity presumably at least as complex as all of nature (since god is normally supposed to have made nature) to nature, and thus very much complicates one's hypotheses.
Now, none of this is a refutation of the thesis that there is a god or may be a god, but it is a good argument to the effect that one should avoid the hypothesis of a god or gods if possible.
3.2. The argument from the previous section is strengthened by the fact that there seem to be at least 3500 conflicting religions, of which at most one can be true, whereas most religions seem to insist that the god or gods of their faith are fundamentally mysterious.
Now here are two relevant points about mysteries. First, there is much that is mysterious, with or without gods, whatever one believes, since there is very much that human beings do not know. Second, a mystery is a lack of knowledge, and a lack of knowledge cannot explain anything but that lack of knowledge: To claim that there is a God and His ways are mysterious is to invoke a possible explanation and withdraw it in the next breath. It is good rhetorics, but bad logic and no explanation whatsoever: What one does not know, one does not know and that is the end of that. And besides, it does not seem quite fair or honest, this hiding behind God's mysterious nature: If you do believe, then why not stick out your neck, and say positively what it is that you believe, so that you run the honest risk of being found to be mistaken?
3.3. A final argument here that should be considered by believers concerns the fact that some very intelligent men, such as Voltaire and Benjamin Franklin, were theists: They believed that there is some sort of maker of the universe, but also that extra-ordinarily little is known about this maker of all, except that he (or she or it or they) is not as ordinary religions dogmatically claim (because ordinary religions are just incredible and contradictory), and it is also not likely that he (or she or it or they) is much or at all concerned with human beings.
This line of argument probably gives comfort only to intelligent men who were raised religiously, and who have rejected ordinary religions for good reasons, yet still want to hold on to some sort of divinity, e.g. because they believe that there is something to the design argument or because it makes them feel better if there would be a god or because they believe it would uphold human morals if there were.
The reason to list it here for your consideration is that, while I have not much of a taste for it, since I had no religious upbringing that disposes me to think or feel positively about god or about gods, and I do not believe that the design argument makes sense, it does seem to me to be about the only form of faith in divinities that may have some minimal credibility, basically because it is a minimalistic faith that does assume far less than the ordinary religions.
Even so: The previous arguments do hold against it, and personally I find it far more plausible to believe there is only natural reality as we know it - billions upon billions of stars and planets and systems of these, conceivably all holding complexities as large or larger than we know to exist on the comparitively almost infinitesimally small earth - about which we know far less than we would like to know, but far more than has been delivered by any known system of human religion, and that for all that we know may have been existing in some form forever, and may exist forever.
3.4. In any case, it seems to me that every honest and intelligent religious believer should admit that he cannot prove his faith or the existence of his god or gods, and believes in his faith basically because of wishful thinking and the peace of mind, well-being, or emotionally important apparent certainties this brings - and not because he has carefully sifted through all the evidence, or indeed considered all religions, or can derive his faith from natural science or indeed consistently combine it with what he knows from natural science, since most religions, and certainly all major ones, are at odds with some of the theses of natural science as developed since Galileo.
And it seems to me that those religious believers who do not admit this, are either unintelligent or uninformed or fanatic - or else insincere. It often is difficult, in the case of many religious believers of many religions, to say which of these alternatives holds for them, while often some or most of these alternatives hold in various degrees, but it should also be admitted that clearly many leaders of religions have been shown to be insincere, and to have professed religion falsely, because of the power it gave them over sincere but possibly less intelligent or less informed true believers.
3.5. Hence in conclusion, it seems to me that all intelligent and sincere and moral human beings should not impose their religious faith on others; should not claim their faith is the true one; should not persecute non-believers in their faith; and should wait till they have died to find out what is the real truth about religion, if there is anything to find out then - as the faithful of all faiths have claimed, but have neither proved nor made plausible to the vast majority of non-believers in their particular faith.
In this life, all human beings live as part of nature; the supernatural should only be a serious subject if one has died, and found that, miraculously, one is still there. Until that moment has arrived, religion is most probably vain and false wishful thinking, that may help one to make sense of one's confusions and fears, but not in a rational way, for there is no rational religion for the same reason as there is no rational nonsense: These are oxymorons.