These numbers are highly exaggerated. For example, the largest christian church has more than 1 billion members (member=baptized) and I don't think that currently there is any way of officially leaving the church after being baptized...
Of course, most people don't bother even if they aren't actually religious, so the Church still claims 80% of us are Catholic, even though many don't even get married in a religious ceremony anymore, much less attend mass.
[1] https://en.wikipedia.org/wiki/Apostasy#International_law