 The BBC can be bashful about the place of Christianity in our national life: it recently sanctioned the substitution of the lovely terms Before Christ and anno Domini by the thumpingly prosaic and Welsh examination boardesque Before Common Era and Common Era. Yet Rev, one of its most successful sitcoms, is a little Pilgrim's Progress. Its star and co-writer, Tom Hollander, told me that he considered theming the series along the lines of the Ten Commandments, but it did not work structurally.  Instead, each episode is a self-contained morality play. Last week, the subject was charity. The Christmas programme will be about forgiveness. although the series was filmed long before the St Paul's protest, the characters and struggles of conscience were prescient. Many situations in life could end in the question: what would Jesus do? The fictional vicar examines the limits of his responsibility towards the oddballs and the rarely washed who pitch up at the vicarage - just as the clergy at St Pa 