The outgoing Archbishop of Canterbury was encouraging people a few days ago to read the Gospel of John. I'm not aware that David Cameron makes a habit of this, but he seems to have followed Rowans advice and done exactly that, quoting Johns gospel in his Christmas message.
Nearly every media source headlines on the Christian content of Camerons words e.g.
'David Cameron cites Gospel of John' (Guardian)
David Cameron quotes from Bible (Huffington Post)
Camerons' olive branch to the church in Christian Christmas message (Telegraph)
here's the full text of the speech and this is a much quoted snippet
"But Christmas also gives us the opportunity to remember the Christmas story - the story about the birth of Jesus Christ and the hope that he brings to the countless millions who follow him."