God, in the sense the name is used by theists, is the Creator of the universe. If the creation and continued existence of the universe can be entirely explained without reference to a higher intelligence and there is no "signature" of a creator, it would be unreasonable to believe that there is a First Cause in the sense Thomas Aquinas used it. In other words, Carl Sagan's statement that the universe is all there was, is, or ever will be is essentially correct. Unexplained phenomena, even supernatural beings, may exist, e.g, poltergeists, but they are essentially irrelevant to the physical universe.
In a "closed" universe, there may be room for supernatural beings, but not for a Creator God in which theists believe.