Good! finally someone who knows the definition of "God". What is it?
Your question is inappropriate.
"not a thing" is the Old English nān þing "no thing" (the indefinite article is intrusive as nān/no was only originally used before consonants.
The Þhing was the deliberative Assembly of the Anglo-Saxon people and by extension came to be used for matters and subjects that were "Thing worthy" of being considered in legislatures and judicial assemblies, or indeed any matter that should be considered.
Saying something was nān þing in Middle English was to say in was it something not worth considering, reckoning, or mentioning.
So "God, by definition, is not a thing" is stating "Who cares? it doesn't matter"
Cordially,
Cordially,