I was raised Christian, went to church every Sunday and believed as a kid. Then I stopped believing in all my imaginary friends. Being slightly cheeky, but also I genuinely just grew out of it as I learned more about... Everything, really.
The English speaking world as a whole is still, to my knowledge, majority Christian even if not actively so, so it shouldn't be much of a surprise that many of us came from a Christian background. Nor, I suppose, that so many of are ex-.