It means that although he is apparently a protestant, he is married to a Roman Catholic, his kids are educated in a Catholic school, and it has been suggested that he will become Catholic after he ceases to be Prime Minister.
The prefix crypto- = hidden, concealed, secret ...
Errmm - I don't think that sounds quite right. I didn't mean to suggest that this would happen in the same way that the coach turned into a pumpkin on the stroke of midnight, or werewolves go hairy at the first glimpse of a full moon.
If he does, will the next incumbent of No. 10 have it exorcised?
I second panj's interpretation.
It is believed that he has been 'taking instruction' and is quite interested. His grinning persona is a staple of the new "Alpha Course" in the Church of England which apparently looks on Catholicism quite kindly.