Because hymns even today tend to be learned by ear, I think they may still often qualify as folk songs, but the most obvious folky source for traditional hymns that may have worked their way into the wider culture is the most popular Shape Note hymn book, the Original Sacred Harp. My favorite is page 122, All Is Well, but a careful trip through this work alone will be very fruitful, and even provide contradictory attitudes in a few cases. If you don't have one, call or write Hugh McGraw in Bremen, GA and he'll sell you one cheap!