It looks like 12th or 13th century latin, maybe taken from something liturgical, a hymn; I didn't find it in the one index of first lines that I checked, but there are larger ones to look in. Highly unlikely that the latin was written by Cat Stevens: it is too medieval and has too many images that are part of the medieval theological common stock about the last day. What were the popular sources of medieval latin he might have seen? A latin hymn book? Helen Waddell's books were around a lot, but I don't recognize it from there.