The Christian God exists in two dimensions of time, by definition being uncreated. The universe exists in only one dimension of time, which Stephen Hawking, George Ellis, and Roger Penrose tell us must have begun at the moment of the Big Bang. Therefore the universe must have been created at the beginning of our dimension of time. Most scientists agree that the universe had a beginning. It is possible that some multiverse (super-universe) created our universe, but there is no observational evidence to support this idea. Despite the lack of evidence, most atheists who know anything about cosmology believe in the existence of some kind of multiverse.
The logical conclusion is that God must live outside of our dimensions of time and space in addition to living within our dimensions of time and space. The unstated assumption that God cannot live both within and outside space and time simultaneously is false. Since science tells us for God to exist and to have created the universe, He must exist in at least 11 dimensions of space and time (the four within the dimensions of our universe and at least 7 outside of those dimensions). Therefore He is extradimensional, and probably not composed of ordinary matter (12), but this does not make Him non-existent. The Bible is also clear that God exists both within our universe and outside of it (1).
Most skeptics believe that the Christian God looks like the old man in the picture Michelangelo created for the Sistine Chapel. Yes, this image of God was created through a person's imagination. However, the Bible says that God looks nothing like this image, but is, in fact invisible and cannot be contained within the entire universe. Obviously, it would have been difficult to depict such a God, so Michelangelo decided that old men were much easier to draw.
The God of Christianity also differs from the gods of the world's religions in terms of His nature and existence. Most of the religions of the world describe their god as existing within the universe. In many cases, these gods even have parents. Both of these attributes are logically inconsistent with the reality of our universe. Only a God who is transcendent to both space and time, like the God of the Bible, is logically possible.