But a massively powerful being is not a god.

An Artifical-Intelligent UFO at the enter of our galaxy controlling everything at the quantum level is not a 'God'. Massively powerful yes, but no god. The whole "conceivable" 'God' thing doesn't work, as the very definition of 'God' is being unconceivable and he is a supernatural being thus cannot be measured by any empirical test, therefore, he doesn't exist.

Also, the denial of existence of a God is not a rejection of a God, as there must be an existence of a God to reject their existence.