Defining Religion

Recently the issue of what constitutes religion and how to follow that religion has been troubling me. According to Dictionary.com, religion is defined as the following:

1. A set of beliefs concerning the cause, nature, and purpose of the universe, esp. when considered as the creation of a superhuman agency or agencies, usually involving devotional and ritual observances, and often containing a moral code governing the conduct of human affairs.

2. A specific fundamental set of beliefs and practices generally agreed upon by a number of persons or sects: [i.e.] the Christian religion; the Buddhist religion.

3. The body of persons adhering to a particular set of beliefs and practices: [i.e.] a world council of religions.
(...)
6. Something one believes in and follows devotedly; a point or matter of ethics or conscience: [i.e.] to make a religion of fighting prejudice.

There seems to be several ways of defining religion. It could be a code of conduct written by people for society to live by. It could also be a set of doctrines that one person uses as a guideline to living his/her life. It seems then that a code of ethics set by a branch of science for scientists could also be a religion. The scientific method is a sort of ten commandments.
The question that I pose by the above interpretations is this: can a person live an entirely non-religious life? That would mean not having guidelines of any kind to living. What would result is a hedonistic lifestyle, or perhaps a sort of paleolithic or neanderthal survival instinct culture. (This largely depends on the resources available. However, hedonism could be considered another sort of religion.) Does that mean life without religion would mean a reversion to a lifestyle resembling prehistoric man? According to Mithen, the cognitive fluidity required for religion is nonexistent in Neanderthals (pg 233). Perhaps because all humans possess the required neurological characteristics for superstition and the ability for cognitive mapping between the perceived and abstract (Fauconnier), living non-religiously is entirely impossible (assuming no brain damage or cognitive disability exists).
Opposite of non-religion is following a set of codes too closely, which could result in misinterpretation and/or prejudice. Consider the prejudice inherent in the Koran against the infidel, and how literal interpretation has led to martyrdom as a subculture in the Islamic faith (Harris). Islam dictates every aspect of Muslim culture, which is acceptable according to the first definition of religion above. However the problem with such religions develops when doctrines sanction violence as a way of promoting that religion. This is what makes me question following a religion so closely, and whether a non-religious lifestyle is possible.
Where is the middle road? Does religion dictate humanity or does humanity dictate religion? I believe both occur, and that the degree of religion dictating a person’s behavior should be left to the individual to decide. Therefore my decision is never to follow a religion like Islam, but to keep the moral codes I have learned in Unitarian Universalism and the scientific method in the background.