I was finding this quite interesting until you went and ruined it with your final sentence/question, Gromit.
There's no justification for an insinuation that these missing Christians are as a result of an increase in Islamic worshippers.
Personally I think it's all those Happy Clappers ramming it down your throat which puts a lot of people off, what with suddenly "finding God" (where had he been hiding, I wonder?).
Apart from the HC set, I think it's simply "not cool" to lots of people being religious these days. They've always got better things to do, seemingly.