Ok, you describe your perception of reality as you personally understand it now, I think you would agree that there is more to it, and that the sum total of all existence, known and unknown, is beyond human conception. But the sum total of all that exists, actually does exist. Over the ages, different times, different cultures, mankind have come up with religious teachings have evolved that are meant to help people find meaning to their life. These religious teachings have a name to represent the source of existence, the creator, the preserver, the destroyer, universal awareness, etc., God, Brahman, Tao, Nirvana, Allah, etc., etc..
Now the name Brahman for example is a concept meant to represent the reality of all that exists, known and unknown, manifest and unmanifest. How does an agnostic doubt that Brahman exists? Now I am not talking about any other aspect in scripture attributed to Brahman, just the reality itself represented by the concept, the universe no less.