According to the dictionary, religion is defined as a "system of worship". Religion can occur if people have faith in something, be it an idea, an object, or a person. These are just broad non-specific definitions. The truth is there is no tight detailed explanation for what religion is. Out of three possible definitions (ultimate concern, transcendent, and ethical dimensions ultimate concern is probably the closest and most valid theory to defining religion.
The ultimate concern theory covers more than the mainstream religions in definition. It also includes groups of people who follow one set of goals or beliefs, even if these practices are not "pious". Some of these groups are the Communists, Nazis, Humanists, and Socialists.
In my opinion, all of these groups are in the same category as Christianity, Judaism and Hinduism. If religion is a "system of worship" then all of these are religions: people worship the ideas and uphold the beliefs of their system. Nazism, for example, had a huge following, and
people constructed their lives around the rituals of Hitler: that should be considered a religion. Hitler introduced a fascist way of life, and the people let fascism control their lives. Certain actions and behaviors emerged from this belief in fascism; a very predominant one was to loath the Jews. The behaviors of the Nazis, and the many followers constitute a "system of worship," or a religion.
Money controls people, and is worshipped as an object and as an idea. People shift and change their lives just to achieve an economic nirvana; this should be considered a religion. All of these groups, objects and ideas all motivate people to do certain things or to act a certain way. These groups all have some kind of rituals, tangible or intangible, physical or emotional. All of these descriptions match those of Christianity, Judaism, and the other mainstream religions.
Religion should NOT be defined by ethical dimensions. Theoretically, there is no way to argue what is ethical and what is not. Ethics is an agreement by the majority of the people, and not always what is "right". So, there cannot be a hard explanation for what is ethical because ethics has changed considerably through time: 120 years ago slavery was ethical and 200 years ago the guillotine, as a method for the death penalty, was ethical. Therefore, it is a social understanding that all religion must be "good" or "holy". Religion can be bad, even in this world of defined morals. If religion is defined by good ethics, then it is contradicted by Satanism and Paganism. These two cults are religions; they however do not agree with any standard of ethics we hold today. Satanists worship the devil and evil, which is highly unethical to God-fearing people. Ethics is a very broad term, left to personal interpretation and should not be used to identify religions. In the eyes of the Communists and the Nazis, their practices were ethical; so how can we deny that they have a valid religion?
A higher power does not make a religion. It is not proven and impossible to prove that a religion must have some higher power: a person, object or idea that is beyond reality. Transcendent thoughts are mere opinion and it is impossible to prove that an opinion is completely correct; thus makes it an opinion. Many people believe that all religions must have a "God". However, there are no concrete facts proving that all religions must have higher powers that people can worship; it is just majority opinion.
In conclusion, I believe ultimate concern is the best broad definition for religion. It has the most possibilities and is the most liberal belief. We will never achieve a specific definition of religion, which takes validity away from the transcendent and ethical definitions. Systems such as Nazism and Communism have no higher power or ethics (in the eyes of the majority), can still classify as religions because of their following and the accordance with ultimate concern.