Originally a French Christmas carol, adopted into English by James Montgomery who wrote the lyrics; arranged as an lively Greek Bouzouki instrumental in Tsifteteli rhythm.
I have always believed this chant to be of some vague 'Latin' origin. In my head it always sounded the way you hear it here: in a powerful Tsifteteli (Belly Dance) rhythm with burning Bouzoukis. My surprise was great when I discovered that it was actually a French Carol. That's why the synthesis was possible and natural after all.