Iain Duncan Smith is a Catholic. (The same goes for the last LibDem leader, Charles Kennedy.) In practice that wouldn't have kept him out of Number 10, if there'd been a Tory majority under his leadership.

Michael Howard's religion is of course irrelevant in this context - there's no kind of ban, even in theory, against Jews, Moslems, Hindus or any other religion or non religion. It's just Catholics.

I've sometimes thought that a good way to deal with the Northern Irish entanglement would have been for the Queen to turn Catholic. So much for the appetite for the English connection so far as Ian Paisley was concerned, I suspect...