Everyone goes out and finds christians, tries to deconvert them in a reproducible way, and then submits some evidence that they succeeded.

That might not be sufficient. The amount of effort to change a believe is often proportional to the strength of the belief after the change.

You do not want someone to react to your statements with 'yes, you are right, whatever you say honey'.

Also it is not particularly rational to have more people following beliefs that are currently in fashion. Communist countries had high numbers of atheists, but for what prize.