Back to the church, and our assertion (note the plural: not necessarily mine) that "we believe in
one God ...etc". What is meant by "God"?
The focal point within Christianity is the first words of the bible "In the beginning God created the heaven and the earth", repeated so as to link with Christianity as "In the beginning was the Word and the Word was with God and the word was God" in the late "Gospel of John". The universe is/was created by God, and Christ is fundamentally connected with God. It is god who ultimately creates "Isness".
I have a problem, however.
For me, as an erstwhile physicist, what we call "the universe" is the totality of a complex 4-dimentional space-time, governed by the rules of physics. In it we live as a tiny speck.
We can explore the universe through telescopes and discern, at distances that science is steadily increasing, the galaxies (in one of which we live). And, as said in "the hitchhikers guide to the galaxy", the universe as a whole is seriously big!. But if God created it, God's existence extends beyond the universe: not in the sense of measuring, but as an extension of the nature of being.
This is the ultimate mystery.