Let's assume that by word "God" we mean the master or creator of everything including itself. Now, if such GOD existed how could it ever be confident that it is the origin and is of everything. Perhaps such GOD would have at least a thought that there may be hidden realms out of it's perception and control. So no matter how powerful and all-knowing a God may be there is no guarantee that it is the essence of everything. Also, the concept "all knowing" maybe nothing but utopia in the end. Therefore, proving that something is God is futile.