Here, lightly paraphrased, are three assertions I found in a Kindle book from www.edge.org (What Have You Changed Your Mind About?).
When thinking changes my mind, I'm a philosopher.
When a god changes my mind, I'm a theist.
When facts change my mind, I'm a scientist.
About a week ago an xian I know told me atheism is a religion. I asked him for his evidence and he right away backed off, saying he didn't want to debate the matter. I told him an assertion made without evidence can be denied without evidence, and that he and I had a standoff.
Thinking about our exchange and then seeing the above led me to devise this question: When X changes my mind, I'm an atheist. What is X?
I'm recalling the time in a Catholic school religion class when a nun said doubting my faith is a sin, and I protested (to myself) that unless I doubt I'm unable to think. Is doubt the X? Skepticism?
Perspective. Context. Non-consensus.
This was going to be my answer as well.
For me X was the wall of doubts that became to high for me to scramble over any longer. So yes, I agree with Len's answer (reality).
I'm afraid the question doesn't really make sense. In all the other cases (philosopher, theist, scientist) the final position derives from the method (X) that is particularly relevant: I choose God (X), therefore I am a theist. In the atheist case there is no method that is peculiar to atheism. Atheism isn't a position that reflects a certain unique method, rather it is a belief (or non-belief) that derives from the successful application of two existing methods: 'thinking' and 'facts'. I would say something like: when 'thinking' AND 'facts' change my mind, I'm an atheist (because atheism derives only from those two things). I don't think there is any particular way of thinking or object/theory etc. that leads to atheism.
I don't know if I explained myself very well...