The fundamental answer to this is
Ockham's razor: don't assume concepts you don't need. Or, even more fundamentally: concepts are only 'placeholders' to express facts about reality, so it is fundamentally unsanitary thinking to
assume a concept as given and reason about its existence afterwards. Every meaningful concept reflects observations in reality.
So if God does not manifest himself in reality, God is an ill-defined term (somewhat comparable to Bertrand Russell's the present king of France) and anything you want to say about it is meaningless.
Another way of putting this: existence means manifestation in reality. If God doesn't manifest himself in reality, he does not exist by the very meaning of the word. Again, this is only true when you look upon existence with a scientific frame of mind.