Seems to me people lose track of what is meant by "hymns", "carols", and "a spiritual".   

Hymns are, by definition, religious songs of praise to some entity, in our culture a god. If it's an atheist occasion, a hymn would be entirely out of place, don't you think?

A carol is a folk or folk-type song (not amounting to a hymn), associated with a particular holiday.

Dave Oesterreich