He is a Christian as it happens, and takes it quite seriously. He's made a recent speech where he's one of the few major western leaders to speak out about the persecution of Christians in the Middle East. People have a very warped idea of the man from the media. He believes in good will to all men of good will, regardless of faith, but his own faith is Christian.
Yes, I understand that the King is supposed to be the head of the Church of England, but the Church of England has abandoned Christianity also — so in the famous words of our former Secretary of State and erstwhile Commodity Trading Savant, ‘What Difference does it Make?’
It makes a lot of difference to a lot of people. Prince William's recent marriage in a traditional Church service at Westminster Abbey is quite possibly the most watched Christian service around the world in the last decade. The Coronation as a Christian service (and it is) would be even more watched. I agree that the Church of England has lost its way to a large extent, which is one reason I've left it myself (an option far less available to the Prince of Wales, otherwise - that's as far as I can go with that thought!), looking for a better way - but despite the failures of some of the leadership, there's a lot of devout Christians still within it. And these big national services help to remind people of what the Church should be - and to remind Britain of what it should be.