I'm atheist toward all Gods that I know of. Every definition of God that I've encountered so far relies on supernatural powers and explanations and thus is not logically cohesive with my naturalistic world view. I will not define God and the idea that an atheist must define the God to which they are atheist is akin to asking someone who isn't a football fan to name the football team to which they are not a fan or ask them what football is. Atheist is more a term of convenience to quickly, and hopefully effectively, convey my attitudes toward religion and colloquial meaning of the term God, which is most typically the Judeo-Christian deity, it is not a philosophical catch-all. If I were forced to define myself philosophically, I would only defend a stance of agnosticism, but for practical purposes, I am atheist.
While God and religion are certainly used to influence the behaviour of people, I believe that it historically developed out of a desire to explain phenomena and a tendency to do so by anthropomorphizing said phenomena. Eg. That the sky can be angry and blow huge gusts of wind and that it must be appeased so that it will bring the rain, as though the sky were analogous to another person.
Considering our ancestry as social primates with strong emphasis on empathy and understanding the motives of others, it is perhaps not surprising that we would ascribe human-like spirits to what we now know to be inanimate and nonsentient objects. Combined with our proclivity to problem solving and toolmaking, pattern recognition to the point of superstitious fault, and natural social hierarchy it seems destined that we would create an entity that gives us purpose, as all our tools have purpose, explain and govern natural phenomena, due to confirmation bias and natural aversion to risk (raindancing as a form of Pascal's Wager), and would sit above all humans in the hierarchy as the ultimate alpha.
Following a progression from animism, where every rock and tree is a person, to deification and polytheism, where larger groups of phenomena determined themselves to be nonsentient are encompassed by a thinking, feeling person, to modern day monotheism, where the entire universe is encompassed by a single person but one who still passes moral judgement and with whom we may still speak and interact. Of course, hindsight is 20-20.