I’ve shamefully adapted much of the text below from Wikipedia.
O magnum mysterium is a responsorial chant from the Matins of Christmas. It has an associated plainchant melody, and in this form has been sung since the middle ages. However, the text has appealed to many composers over the years. Many of the most notable composers of the renaissance made settings, including William Byrd, Jacob Clemens non Papa, Cristóbal de Morales, Palestrina (article on the setting) and Tomás Luis de Victoria. The famous version below is by Victoria, who went on to publish a mass based on his motet in 1592.
Latin text
O magnum mysterium,
et admirabile sacramentum,
ut animalia viderent Dominum natum,
iacentem in praesepio!
O beata virgo, cuius viscera
meruerunt portare
Dominum Iesum Christum.
Alleluia!
English translation
O great mystery,
and wonderful sacrament,
that animals should see the newborn Lord,
lying in a manger!
O blessed virgin, whose womb
was worthy to bear
the Lord Jesus Christ.
Alleluia!
Painting of the Nativity scene, by Hans Baldung (1520)
I always associate Victoria with a much loved uncle who, sadly, died at the age of 60—he gave me a record (!) of Victoria’s music when I was a boy. For contrast, here is Palestrina’s setting. This very famous setting merits its own article at Wikipedia: O magnum mysterium (Palestrina):
And we finish with a lively psalm setting by Adrian Batten:
Lauridsen wrote a beautiful setting as well: https://m.youtube.com/watch?v=lO5MheI0A8o
Magnum Mysterium gave me chills. Thank you. Sharing another that I love: https://youtu.be/3s45XOnYOIw