I really wouldn't call Atheism a guess. An atheist looks at religion and says " There is no evidence to conclude this deity exists, therefore I will not believe in it, or any other. It's like anything else. I do not see a red ball on the floor, so I can conclude there is no red ball on the floor.
Theism is a guess based on faith.
That works both ways. A theist can say "There is no evidence to conclude this deity doesn't exist, therefore I believe in it".