Christian and Atheism Worldwide Demographics: Current Realities and Future Trends.

[Belacqua]

Numbers aren't proof but they are often evidence.

But evidence needs to be tested.  Ultimately all the numbers do is suggest "This appears to be a rather popular hypothesis.  Let's design an experiment to examine it in more detail."

Yes, evidence points in a certain direction, but does not constitute proof. We should be skeptical of evidence to the degree it's warranted and test what can be tested.