/me shrugs
I'd give the argument from evil. If there were a God like the one that Christians worship, the world wouldn't suck as much as it does.
That does not invalidate the "argument" in any way, since mysterious ways include the sucky world, and in any case, it is all just a "test".
These are written from the angle that Christians deconverting is bad.
Cognitive reasons
Breaking up because a relationship with God becomes unworkable
Leaving because other Christians aren't empathetic about doubt
At what age do people leave?
It looks as though there may be more articles in the series.