I might add that I've performed medieval sacred music in a church as part of a service - if you can call the Cantigas de Santa Maria sacred. At the Morpeth Gathering one year I did a short set of stories of Saint Cuthbert (from Bede) as part of a Sunday service and had the congregation singing along with Gelineau Psalm 109 (110). As an Atheist I see no problem with this whatsoever; indeed, it was an honour to do so. I had the words of Ca' The Horse Me Marra printed up in the Order of Service and the congegation sang along with that too. Awesome!