Let us say there is a difference between real and existent mental objects - for example the Unicorn which has existence as a mental object, but is not real (ie does not correspond to physical reality).
Now God, in various different forms also has existence as a mental object, and according to the accuracy of that object may be defined as real.
Suppose I say that God is all - God necessarily has existence, as an idea, and is real because the definition contains no logical contradictions.
So if someone were to define God clearly enough to avoid any disparity between the mental object and reality, he must by necessity be real.
So when the atheist attacks God, he is really attacking the mental object of the believer, which is usually badly defined.