Full Idea
Aristotle is committed to the view that strictly speaking there is no proof of the essence and existence of God. There will be a real definition of him as an axiom of special theology, and then a deduction of theological theorems.
Gist of Idea
For Aristotle God is defined in an axiom, for which there is no proof
Source
report of Aristotle (Metaphysics [c.324 BCE], God) by Michael Frede - Aristotle's Conception of Metaphysics p.94
Book Reference
Frede,Michael: 'Essays in Ancient Philosophy' [University of Minnesota 1987], p.94
A Reaction
The cynical response would be 'why not start with a Great White Rabbit, then?', but I presume one must read what Aristotle says (late in 'Metaphysics') to understand why this particular axiom is chosen. Economy, power etc.?