Religion is a term which does not have a very good definition. Attempts for it can be found in the literature. For example, the father of sociology, the philosopher 
Emil Durkheim, 
defines religion as
a unified system of beliefs and practices relative to sacred things, 
that is to say, things set apart and forbidden - beliefs and practices 
which unite into a single moral community called a Church, all those who
 adhere to them.
 My fellow blogger 
Sabio spends quite a bit of blogging energy to elucidate this point. I am not a scholar of religion, neither a philosopher, and certainly not one who would devote time to come up with definitions of religion. (I don't have much time to waste.) Nevertheless, I think I know what religion is when I encounter it.
I am talking in rather vague terms because "religion" is not just a system which calls itself a religion. Example of these systems are Christianity, Islam, Judaism and Zoroastrianism (all of which have the common feature of being monotheistic). It is also a system which resembles what one traditionally would call a religion. For example, the North Korean 
Juche is a religion. When people blindly obey someone else's orders and have reached a point where questioning them is inconceivable, then this is religion.
One can argue, based on what almost all religions proclaim, that the main requirement of every religion is the concept of belief. Now, the word itself can have many meanings. Religious belief, however, can be characterized rather precisely. After all, all religions require belief. They require 
faith. Have faith and you shall be saved! Have faith in god. Tony Blair established his 
Blair Faith Foundation in order to teach how religion influenced his politics. 
Religion requires belief. Religion requires faith.
Belief is a complex concept. It is a word that can have many meanings. But religious belief has a very important feature making it uniquely distinguishable.
Religious belief about a concept X is an increasing function of the amount of evidence against X.
That is, the more we learn about the ridiculousness about X (say X=miracles), the stronger the religious belief about X is. 
But belief is the weakest point of any religion. What religious people cannot get is that any time one is asked to believe without asking, without investigating, then they are possibly being tricked into something untrue. I grew up understanding that the concept of belief is terrible. I can, of course, temporarily accept something in order to go on, but I have to question it at some point.
People believe for various reasons. One is that belief is easy. It is much easier to convince yourself to believe rather than to understand, for example. Because believing takes little work. But understanding often takes a lot of hard work. Any rational person can 
obviously see the flaw of the (religious) belief concept.
When I teach I ask my students not to believe me. This is tough. Students have been conditioned on the idea to believe their teacher. Again, this is because it makes life easy. A student is tuned to believe that a continuous function, which takes value 1 at the beginning of an interval and 2 at the end, must take any value in between. Sure, the teacher says so. But a proof is needed. Belief is the easy thing. Proof is, typically, the hardest.
In the recent years, a number of so-called apologists (mostly of the Christian kind, but there are Muslim too) have  sprung. See, e.g., 
here and 
here and 
here. There are many examples of that kind. These are people who appear to be intellectual and are typically holders of advanced degrees, awards, have done serious work on some (non-religious) subject but, at some point, go crazy and start arguing that (their particular version of) religion is explained via logic, science, empirical observation. And they have lots of supporters. Sure, people are thirsty to believe; to believe that their religion is explainable because this is what these famous, important, outstanding public "intellectuals" advocate. They become the heroes of the unthinking masses. Despite all their attempts to "scientifically prove" that belief is not the main characteristic of religion, they fail, and fail badly. Religious belief remains religious belief and none of these pseudo-intellectuals have provided a gram of evidence against it.
Jesus
 said to him, “Have you believed because you have seen me? Blessed are 
those who have not seen and yet have believed.” [John, 20:29] 
God
 will test believers to make sure they believe and are afraid. If they 
fail a second test they will suffer a painful punishment: 
 
O  you  who  have  believed, 
Allah will  surely  test  you  through  something  of  the  game that  
your  hands  and  spears  [can]  reach,  that Allah may  make  evident  
those  who  fear Him  unseen.  And  whoever  transgresses  after  that  -
   for  him  is  a  painful  punishment. [Quran, Sura 5:94]