The quick and easy answer to why people are religious is that God—in whichever form you believe he/she/they take(s)—is real and people believe because they communicate with it and perceive evidence of its involvement in the world. Only 16% of people worldwide are not religious, but this still equates to approximately 1.2 billion individuals who find it difficult to reconcile the ideas of religion with what they know about the world.
Why people believe is a question that has plagued great thinkers for many centuries. Karl Marx, for example, called religion the “opium of the people”. Sigmund Freud felt that god was an illusion and worshippers were reverting to the childhood needs of security and forgiveness.
A more recent psychological explanation is the idea that our evolution has created a “god-shaped hole” or has given us a metaphorical “god engine” which can drive us to believe in a deity. Essentially this hypothesis is that religion is a by-product of a number of cognitive and social adaptations which have been extremely important in human development.