Some will ask, "Are we talking about a god or gods?" Is God one and monotheism true, and if so, is the Supreme Being personal (possessing intellect, emotion, and will)? Or is the divine being contained in or identical with all that exists as believed by Eastern monists (pantheists)? What exactly do we mean by the term "god" and how does the proponent of a given concept know that such a concept is true?
We can start with our existence and the world around us. Physics seems to point in the direction of a "finely tuned universe" that can be described mathematically by human intelligence. This could suggest that the universe is the product of a purposeful, single intelligence. (See The Anthropic Principle and the Intelligibility of the Universe.) The intelligence behind the universe could also hint at the idea of a purpose, will, or volition. The very act of creation implies volition. Intelligence and will are two components of personhood. The third dimension of personhood, emotion, can only be experienced through encounter or what the Jewish theologian, Martin Buber called the "I-Thou" relationship involved in religious faith.
The outworking of monotheism as it has shaped our understanding of the world is a further support of its truth. It is an historical fact that the close observation of the natural world occurred in civilizations based on the monotheistic faiths (Islam, Judaism, and Christianity). That the scientific method arose specifically in the Christian west is due in large part to the idea of an all-wise and rational God who created the universe and ordained the regularities of nature.
The classical concept of God does not refer to a being alongside other beings in the universe. When the Russian cosmonauts stated that they "didn't see God out there" they were making a fundamental error. C.S. Lewis commented that such a statement was similar to "Hamlet looking for Shakespeare in the attic of Elsinore Castle". God, who is an eternal, (uncaused, non-contingent) spirit, is not a being whom we can hold at arm's length and evaluate with cool objectivity. Rather, God is the source and foundation of reality, the cause behind the universe, the creator of space and time, the source of all being and consciousness.