If I made up my unique definition of God, you would have a valid point. By using the common definition of God, I'm sticking to what most people tend to define God to be. If I was to write about all the possible alternative definitions of God, that would be a whole book.

People did used to believe the earth was flat. Most people don't now. But if I come up wth my own definition of the word "flat", and say that means spherical, I could prove that the Earth is indeed flat.