Hanne Stinson, director of the British Humanist Association, says she thinks many of them are "cultural Christians".
They see themselves as being Christian in the same way as they are British, almost in a tribal way.
"People label themselves with what they were brought up with," said Hanne.
"If they haven't gone to church for 20 years they still put themselves down on official forms as Church of England.
"Even one of our members put himself down as Christian on the census - it's a common reaction of someone who's been brought up Christian."