That won't happen!
There is, neither definitive proof now, or in the future of His non-existence.
The problem is using a certain definition of the word "exist". If we are to refer to existence in terms of physical, material things, then you could argue that He does not exist.
The problem with that is, how do we explain the "existence" of something spiritual? Does IT exist. Some say yes, some say no.
God IS Spirit.
He manifests Himself, reveals Himself in (often) intangible ways. He is subjective, and He is relational. Not material, physical, tangible.