Devon has a long history of Christian tradition, dating back to the intermingling of Celtic paganism and Roman Catholicism from the 4th century A.D. onward. After the English Reformation, Devon, like the rest of England, became associated with the Church of England. Puritanical strains of Protestantism and Methodism became widespread throughout the region. Many smaller, Anglican churches cropped up throughout Devon in the 18th and 19th centuries, some of which share practices, structures, and belief systems that are like those of the Barum Brethren in The Long Call.

In interviews, Cleeves has specifically stated that the Barum Brethren are a fictional group, but they are based on a very similar, insular religious group from which one of her close friends was “unfellowshipped,” just as Matthew Venn was “unfellowshipped” from the Barum Brethren. Real religious groups such as the Plymouth Brethren (also called the Assemblies of Brethren) share some of the qualities of the Barum Brethren. The Plymouth Brethren began in Ireland in the 1820s and went on to hold meetings in Devon in the 1830s. Shortly thereafter, the group underwent a schism, resulting in the Exclusive Brethren and the Open Brethren. The Exclusive Brethren, who still practice their faith in Devon today, are a highly conservative religious group.