Yeah, it's obviously Latin - now that I see the lryics printed - but I didn't know it was Medieval Latin.   
I've obviously been "mondegreening" several of the lyric lines. LOL

I think this is one of Cat Stevens' best songs. Too bad it's not as well known as a few of his others.

Thanks, Joe.

PS,
Dammit! I really wish people who put lyrics online would use PUNCTUATION!    Aarrgghh!