The only "churchy" godless hymns I've heard (and sang) are not folk but classical, inspired in medieval (and banned) profanity : Karl Orff's CARMINA BURANA is a cantata composed in 1937 on the basis of authentic poems dealing with carnal pleasure, wine, gambling and everything known to be good in medieval times. Some of the songs and dances could be adapted to pop and result in a pleasant surprise in a folk/pop concert. The melodies are simple and beautiful.
Un abrazo - Andrés