It is impossible for a god to exist.

God is considered to be the most highly-developed being in existence, highly surpassing humans.

Development cannot happen unless it's gradual.

Development is tirggered as a reaction to outside stimuli.

If we consider that at the beginning, there was nothing, that would put a god entity candidate in the same position as every other living being, reacting to the same set of stimuli.

Ergo, a god candidate would be no more or less developed than any other creature at a given point of time.

Ergo, he wouldn't be a god.