I would add that the question is not only affected by the definition of "folk" but also by the definition of "hymn". Christian hymnody through the ages has been influenced by everything from Hebrew canting of the Psalms to Medieval plainsong and chant, in addition to popular or "folk" tunes. I would submit that there is a particular definition of "hymn" which distinguishes it from Gospel music, or spirituals, etc.

In the Presbyterian and Reformed Protestant tradition there were lots of arguments in the 19th century over the use of "Gospel" music vs. "Hymns" in worship. Interesting that what we now consider 'traditional' (In the Garden, Old Rugged Cross, Blessed Assurance) songs were radical in their day -they did not quote the Bible, for one thing, and were very individualistic (note they are all about "me & Jesus" and use "I" a lot) whereas hymns tend to be more communitarian - 'we' and 'us'; and rely on Scripture for reference rather than personal experience. Of course the difference was contextual - Gospel music was born out of the 19th century (mainly American - thus the extreme individualism) revival movements, their rejection of "staid" and "formal" worship, and their emphasis upon a personal, emotional experience of God. In style and content, Hymns have more in common with their Roman Catholic and Hebrew roots.

In form, hymns tend to be written in poetic stanzas rather than the verse-chorus-verse of much folk and popular music. And then there are also the spirituals, with their roots in African musical stylings, and contemporary "praise choruses", which I would argue are an updating of the Gospel, revivalist music tradition. But that's another thread.

I guess I understand the definition of "hymn" more clearly than I do the definition of folk music, so the question remains ....

Peace,

Bo