Could Secularism or Atheism or materialism be considered a religion? Any Philosophy that deals with life and people and how one interacts with all this in order to maintain some sense of a good life and has at it's base a set of principles to deal with any philosophical problem one might encounter could be called a religion. Even evolution theory could be called a religion because it is supposed to answer fundamental questions of existence like "why are we here?".
To a large extent this was inspired by Yannik's answer; but I hope that I elaborate on points differently enough to not be redundant.
As far as I can tell, atheism (or secularism or materialism, considered separately) is a component of an over-arching worldview not the totality of the worldview itself. For example Secular Humanism is a worldview whose breadth and scope is comparable to most religions, and atheism, secularism and physicalism are just a components therein.
I'm inclined to point out how this is similar to the mono/poly theistic division: there are worldviews with many gods, one god, no god but a host or more limited spiritual agents, no god and a rejection of all spiritualism. Worldviews in the first two categories are theistic, and what Westerners usually refer to as religions. Obviously, worldviews that fall in the third or fourth categories have the feature of being atheistic. To me saying "polytheism is a religion" doesn't fit right, I'd say "This particular religion is polytheistic." In this way of looking at things asserting "atheism is a religion" is a category error.
Similar considerations hold for secularism and materialism: these are (or can be) components of any set of worldviews, but aren't themselves in the same category as religions. As mentioned in the first, summary, paragraph Secular Humanism is a world view, comparable to a religion, that does exhibit all of these features.
Despite the general consensus, atheism insnt a world view or a life mentality. Its simply the rejection of theism. So an Atheist can be pro live, reject the theory of evolution and be against premarital sex. The same applies to secularism, if you are secular you only want the state and religion to be separated. You still can be a theist and believe in god. So it doesnt even come close to a world view.
For materialism, you could consider it a religion. Tho you would need to distinguish between religion as a way of living and religion as theism, because people would get confused and think you mean that they pray to money or something ;-)
A lot of this hinges on what one takes to be a 'religion'. The term is not actually all that old in its current usage. (This is one reason why Japanese people imagine that they are not religious - 無宗教 [musyuukyou lit., "non-religious] - because they don't see things they regularly do like 墓参り [hakamairi grave vistations] or お盆 [o-bon holiday in August to welcome dead anscestors to your home] or お正月 [oshougatsu new years -- which involves going to a temple, praying and getting a fortune] as religious). Even in English, the term refers more to a sociological area of study than something else.
The debate is primarily over what religion is versus what religion does.
My sense is that atheists reject the term largely because they think the term has something to do with believing in a god or gods -- against the substantive definition as it is primarily associated with Western religions.
As you suggest in your question, if you define religions as things that try to answer the substantive questions of our lives, then clearly atheism in some forms fits that definition. Similarly, if you define at as generating a coherent life picture, it would fit that too.
But as suggested in a comment above, there are some other candidate terms to consider that are not as confusing as religion: like world-view, life philosophy, or just philosophy. (Here, meant not in the sense that is best applied to what philosophy.se should consider).
Obviously atheism is not a religion. Atheists completely ignore some strange belief system that some people have which causes those people to do all kinds of strange things. There are always people who say they believe in some god and atheists don't, but atheists don't say "I don't believe in god", they say "what is that god thing that you are talking about"?
Since that strange belief systems of others is ignored, it doesn't play any role in the life of an atheist and is of no importance whatsoever (except that from time to time this needs to be clarified to some people). As it doesn't play any role in the life of the atheist, claiming atheism is a religion is quite ridiculous.
See also a quote of Asimov in this thread where he says "I so strongly suspect he doesn't that I don't want to waste my time.".
"considered a religion?" No "Any Philosophy that deals with life and people and how one interacts with all this in order to maintain some sense of a good life and has at it's base a set of principles to deal with any philosophical problem one might encounter could be called a religion." And Atheism has nothing to do with any of that. "Even evolution theory could be called a religion because it is supposed to answer fundamental questions of existence like "why are we here?". No, it's meant to answe the question, "Why are animals different"?
Obviously atheism isn't e.g. the Christian religion, though there are self defined "Buddhist atheists" (e.g. Stephen Batchelor).
One way to answer your question would be to ask whether an inverse or negation of a practice or belief can fulfil the same function, either on a social or individual level.
I can't think of any psychological factor which would make the latter impossible. Or anything phenomenological, though I'd welcome anyone's correction of that from the phenomenological literature on e.g. religion. The imagination is a powerful thing; what is a belief but imaginative conviction?
- Disbelief has content for as long as we consider the alternative.
On a more material level, it's obvious that an atheist can act in any way at all, even attend church services. As a belief, it seems obvious it might fulfil the same ideological role in a collective: provide belonging; self expression (many people hear poetry in Nietzsche); or legitimation (e.g. persecution of religion).
So while atheism isn't any existing religion, and doesn't meet at least some dictionary definitions as it denies any deity, you may (or may not!) suppose it is a religion.
I wouldn't say atheism is a religion, but I have heard it said that the only difference between a religion and a cult is the number of followers. There are definitely groups of atheist who are as fanatical about spreading their beliefs and discrediting others, with a singlemindedness so strong that they rival St. Paul and all he did, especially the people who grew up religious and denounced it later in life. Again similar to Paul's conversion. That is cult like behavior. The Romans referred to early Christians as the cult of Christ, but no history book I've ever seen refers to them in that way. So cult and religion are in the eye of the beholder.