I suspect the phrase comes from a book or programme set in Wales, although it could also be set in some parts of England. In Wales, particularly, the term "chapel" is shorthand for someone who belongs to one of the nonconformist/non-established Christian churches, such as Baptists, Unitarians, Congregationalists, Methodists and Presbyterians. In Wales, "church" means someone who belongs to the Anglican Church in Wales [which is also non-established]; in England, it would refer to the established Church of England.
Of course, all of these people belong to churches, and chapels are not confined to nonconformist worship....