Census 2011: Historian on 'Godless' valleys
3 February 2013 Last updated at 13:17 GMT
Parts of Rhondda Cynon Taf are among the most "Godless" places in Wales, according to the 2011 Census.
Information published this week shows that five council wards in the county are among 12 across England and Wales where the majority of people said they have no religion.
Historian Sir Deian Hopkin was asked by Vaughan Roderick why, on the BBC Sunday Supplement programme.