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...
The right of apostasy is considered part of the ICCPR treaty[1], so it should be respected by all the parties. At least here in Portugal, you have the right to demand they stop using you in statistics and such.
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.
