He's actually speaking the truth. The UK's head of state is a monarch, who holds that her (and her family) were chosen by the Christian God to rule the country. The national religion is Church of England, which is a denomination of Christianity. There are 12 bishops as permanent (unelected) members of the House of Lords. I could go on...but I think it's pretty obvious that the idea of "secularism" as it is properly defined means nothing in the UK, and the UK is in fact a Christian country, much as Israel is a Jewish country, and Iran is a Muslim country.
I of course disagree that it should be seen as a Christian country, but as to what Cameron said; it's all factually accurate.
I of course disagree that it should be seen as a Christian country, but as to what Cameron said; it's all factually accurate.