According to Descartes: for any X, P(X exists | X is taking the survey) = 100%, and also that 100% certainty of anything on the part of X is only allowed in this particular case.
Therefore, if X says they are Atheist, and that P(God exists | X is taking the survey) = 100%, then X is God, God is taking the survey, and happens to be an Atheist.
According to Descartes: for any X, P(X exists | X is taking the survey) = 100%, and also that 100% certainty of anything on the part of X is only allowed in this particular case.
Therefore, if X says they are Atheist, and that P(God exists | X is taking the survey) = 100%, then X is God, God is taking the survey, and happens to be an Atheist.