The simple fact that you are forced to use these silly double negatives in this "definition" of yours only serves to illuninate the innate and deliberate dishonesty of it. And anyway, this is not the definition of an atheist. It's the definition of an atheist's corpse. The definition of an atheist is someone that has rejected the philosophical proposisiton made by theism as being invalid. That IS a consciously chosen position, presumably based on some course of reasoning.
What double negatives? Methinks you are imagining things
The simple fact is your negative bias and ignorance of atheism is showing you up for what you are.
That definition is from the OED, (OXFORD ENGLISH DICTIONARY), you don't like talk to them.