There is actually not as much agreement upon how the word 'religion' should be employed as it sometimes seems. Much of this, I believe, originates in a curiously Western habit of equating three particular and related monotheist religions (Judaism, Christianity and Islam) with religion. In that peculiarly human way, the word religion is applied as a family resemblance category, and therefore anything not related to these archtypal religions is not a religion. To be fair, polytheistic religions are usually accepted as religions by extention, allowing Hinduism and various pagan religions into the fold, but the thinking that leads to this narrow view has severe limits.
I subscribe to Wittgenstein's thinking on the subject of meaning, that the meaning of a word is how it is used in a language. This approach is not compatible with the notion of an absolute meaning of words (and indeed, I do not believe in such meanings), and as such one finds oneself in unusual places once you start applying this principle. After all, there can be as many meanings as people (although I choose to believe that a number of these sets of meanings in each instance can be conflated into common cases). From this angle, I believe there are two broad and basic categories of meanings for the word religion.
Firstly, there is the meaning of religion as it is often employed by lazy atheists and agnostics who often have not identified their agnosticism. Whatever words are used to express this meaning, it tends to presuppose that a supernatural being, force or god is quintessential to something being a religion. This, for instance, means humanism does not qualify, Buddhism is in an ambiguous no man's land, Discordianism is in no man's land and in general, it means that religion is conflated with those beliefs the individual in question has rejected.
Secondly, there is the meaning of religion as it employed by those that practice a religion. There is little in common with the definitions in this set of meanings - but still, I greatly prefer this approach for one simple reason. If this approach is favoured, individuals get to designate whether their belief system is a religion, rather than individuals dictating to other people whether or not their belief systems are religions. The former is polite, the latter borders on passive aggressive. (I must make an exception of those people who say that what they practice is not a religion but a way of life - a statement I have heard from Christians, Hindus and humanists alike... I think anything that is "not a religion but a way of life" should probably be considered a religion by this second approach, if only for the purposes of interfaith solidarity!)
By choosing to side with the second set of meanings, I get to include all the major world religions in my set of religions. Sadly, I am not allowed to include Marxism by this method... despite the fact Marxism seems to me to be an athiest religion with an ethical bias, Marxists do not identify their belief system as religious, and so I cannot allow myself to do the same. In this regard, Marxism is one of those belief systems which could be considered religion-like, if not actually religious. (This is a shame, as it could be helpful for everyone if more atheistic religions identified themselves - some diversity among the secular humanists couldn't hurt).
There are, to my way of thinking, at least two different ways that people can identify themselves with a religion - and both are completely legitimate by any measure. The first might be considered cultural religion, which is to say, when an individual identifies with a religion, but the role of religion in their life is largely to dictate key elements of their cultural framework. I talk to many people about their religious beliefs, and in the United Kingdom I meet many Christians who are ultimately rather agnostic. They are what I would consider culturally Christian, and I have no problem with this. Ethical issues will always be relative unless one adopts a frame of reference to inform one's morality.
Conversely, there are those who follow a spiritual path whose direction is guided by a religion. This might be considered spiritual religion. As a general case, those influenced by spiritual religion tend to be a diverse crowd, even within the framework of a single religion or religious division. People influenced by spiritual religion often talk of being on a 'path', and this conception extends across boundaries of religion. Mystic branches of the major religions favour this approach - such as Sufi Islam (a religion which recognises multiplicity - even in religions), Zen Buddhism(which strives to push beyond the illusions of language) and the Higher Path in Hinduism (a religion which celebrates and accepts diversity of beliefs).
I love and celebrate the diversity of religious practice - both cultural and spiritual. And as such, I feel somewhat sad for those people who believe that by making the decision to be an athiest, they have escaped having to commit any time to learning about their own religious or spiritual framework. Many people in this situation have rejected a very basic definition of god (sometimes one as crude as the "Old man in the clouds" model, which few or no serious religious people would brook), and then just stopped their critical thinking there.
Such people are like angry young teenagers who have grown up in Thiestville and, tired of what they percieve as the inflexibility of thinking inside the city walls, have moved outside and set up camp, proudly declaring "there is no God!" as if that statement wasn't a religious belief. Elsewhere in this metaphorical world, thiests, non-thiests and athiests are living happy lives in different towns, but for some reason these people are content living in a shanty town just outside of this particular town. They reject the way of life inside the town, but do not seem interested in setting out to find a new town to live in. They'd rather sit outside and shout at the people inside town. Sadly, many end up displaying the same intolerance, absolutism and dogmatic fundamentalism that they claim to despise when they see it coming from theistic motivations. They fail to recognise it is not particular classes of belief system which are the problem, but how one relates to one's own belief system, whatever that may be.
A thought experiment I offer to anyone who (probably as a teenager) rejected theism and decided to identify themselves as an atheist (without actually going on to develop a cultural or spiritual atheistic religion of their own). You reject the idea of a god or God, possibly because you see it as untestable - or perhaps you believe absolutely that God Does Not Exist for whatever reason. But do you also reject the notion of the United States? Of France? Of Japan? Of Burkina Faso? What absolute evidence exists for the idea of a particular country? It cannot be tested. People show you maps with boundaries drawn on them, but you cannot go out and measure where these boundaries lie. You cannot detect a country. Perhaps the notion of a country seems more real to you because everyone around you shares this belief in countries (unless you happen to live in one of those places in the world like Chechnya where there is disagreement over geographical belief systems).
I have nothing against atheism, but I cannot abide that kind of smug, self-congratulatory belief system that begins in the absolute assuredness that One Is Right in one's beliefs, and by extention, that one can act against others because They Are Wrong. I do not believe, as many trivially atheist people would claim, that religion is the cause of so many of the world's problems, but rather I feel that the enforcement of absolute belief - be it in God, the absence of God, or the existence or otherwise of a particular country - is the cause of so many of the world's problems. This is a theme I explored in my script for the game Heretic Kingdoms: The Inquisition (although I'm not advocating that you play the game unless you enjoy hack and slash computer RPGs, it should be understood).
I want to close with an example of extreme intolerance. There was a man named Wilhelm Reich, whose beliefs were avant garde to say the least. He believed he had discovered a physical energy he called 'orgone', which he claimed he could use to cure cancer. Needless to say, his ideas were not very popular. He eventually went to jail for his beliefs, where he died. In August 1956, almost all of his books were burned by an agency of the Government which had imprisoned him. Wilhelm Reich was living in the United States at the time, having fled the same treatment in Nazi Germany seventeen years earlier. The people who persecuted and (some say) hounded him to his death were not motivated by religious absolutism, but scientific absolutism. They "knew" he was wrong - and he had to be stopped. I personally have no idea what to make of Wilhelm Reich's ideas - in part, because the US Justice Department burned his books almost fifty years ago.
One cannot, I contend, live without a framing belief system. This may be a religion, or something 'religion-like' (like Marxism), or even an entirely custom built set of beliefs - which is largely where I find myself. But if you do not commit the time and effort to explore your own beliefs, and to explore other people's beliefs, you may be greatly at risk to a kind of absolute thinking which, if not directly correlating with evil, certainly can be traced as a motivating factor behind acts that people choose to call evil.
As for the notion of evil itself, well, like countries, I remain agnostic.
Burkina Faso is a beautiful and diverse country in west Africa. I visited the area in 1981, when it was called 'Upper Volta'... Burkina Faso means "The Upright Land", or "The Land of the Upright People". I was surprised by how modern its capital city of Ouagadougou was, and how beautiful the desert plains and watering holes around Djibo were. I was enraptured and fascinated with its wildlife, especially the antlions and gekkos. I was impressed by the insatiable optimism of a people who have so little, yet live in relative happiness. Despite there being no country of that name when I went there, I still believe in the existence of Burkina Faso.