Christianity is a hugely diverse and quarrelsome family of faiths, but most Christians have nevertheless set great store by orthodoxy - literally, 'right opinion' - even if they cannot agree what that orthodoxy should be. The notion that there is a 'catholic', or universal, Christian faith - that which, according to the famous fifth-century formula, has been believed everywhere, at all times and by all people - is itself an act of faith: to reconcile it with the historical fact of persiste...