This is left over from one of the Advent and Christmas liturgies posted here.
My backing is a bit of a fail (BIAB and chant don’t like each other), but the text is a translation of a fourth century work by Aurelius Prudentius, which is not long after Christmas started becoming an important feast, so I thought I’d mention it anyway.
The tune is Plainsong from the thirteenth century – DIVINUM MYSTERIUM.
All the details including sheet music with no discernible time signature are at Hymnary. Wikipedia has the original Latin.
I guessed some spaces and it doesn’t sound right to me:
This is more like it: