We tend to take other people's word for a lot of things and then find out later that either part or all of what they said just wasn't true. To me, the only way a person can believe in God is by the experiences they have which are spiritual in nature. Taking other people's word for the existence of something isn't a very safe way to navigate through life.
Most people believe in something greater than themselves, and for lack of a better description, call it "God." But many are skeptical about religion and religious beliefs, and therefore, they have different ways of describing their experiences. Still others take the very traditional approach of believing by "faith", which unfortunately in many cases, is based on other people's beliefs and writings instead of their own experiences.