There are 16% of people in this planet still don't belong to any religion!! One may argue animals don't belong to any religion. Wikipedia defines religion as "A religion is an organized collection of beliefs, cultural systems, and world views that relate humanity to an order of existence". The fundamental aspect of religion as per this definition is a belief without any question. One may say religion has brought unity among human being like language did. It also divided as the belief across entire humanity is not consistent.
All religions believe in "God" which is speculated as per the understanding of the experiences. For few people God is some one in the heaven watching all our sins and punishing - hence we call 'God fearing' people. It will always a speculation as God can never be shown in a laboratory experiment as He is beyond the scope of sensual cognition. Einstein could explain theory of relativity only by 'Thought experiment' and not a real one.When we are so indulged in life for sense gratifications, all these God principles appears as futile 'Theory'.
When Bohr proposed Atomic structure or Galileo discovered that the earth is a globe it was a theory as they could not prove these by physical experiment to a common man. They poked his eyes since he could not understand it. Some great thinker said "If you want me to believe God, you must make me to touch Him". Alas what a stupid statement? He also could have said to Bohr to make him to touch atoms or to Galileo to show the globe? Both these great scientists would have failed to convince him. If one has to believe only after cognizing through 5 senses none of the scientific inventions are possible.
When we declare anything as a theory it means we are declaring our ignorance. When touch screen in smart phone was not there will you say 'Make me touch then I will believe?' The moment you are challenging the future possibility you are confirming you are not a 'leader' but a 'follower'. Because a "Leader sees before others could see". Be a leader by believing what is not evident to your physical eyes.