Because there are people that don't believe in a greater power, and then people don't see this great power as being the same thing or having the same properties. Some people believe out of fear, others do it because they were told to since they were very young, and there are some others that do it because they really think it's true.
Then there are others like myself who don't believe because we chose our religion (or lackthereof).
Then there are others like myself who don't believe because we chose our religion (or lackthereof).